Mike (AKA elephantfan85) put this together, and it takes most of the info above and summarizes it into a single Martin timeline.
1920's Martin transitions away from pyramid long bridge to standard long bridge on 18 & 28 series. Happens gradually starting in the early 1920's. 17 series either never had pyramid bridge or transitioned earlier.
1922-1923, Martin makes guitars for Wurlitzer. Some lack Martin Serial #'s or markings of any kind.
1929 18+ series Martins fully switched over to nitro cellulose laquer finish. Transition started in 1926 with O-17H according to Longworth.
1929 Martin switches from rectangle to belly bridge (Longworth 2nd ed page 84)
1930 Oct, first time size & model number stamped on neckblock. Happens sometime after serial number 439xx and before or at 44362.
1932 Some 17 series models transition to 14 frets clear of body & non slothead.
1934 Most models switch to 14 fret clear of body and non slothead. Sometime around here Martin starts putting the Matin decal on the front of the peghead. For a short period of time they continue to stamp the back of the peghead as well.
1934 Martin ends their use of bar frets on most models. (Most other guitar makers had done so around the end of the 19th century.) Martin Hawaiian style guitars retain bar frets until at least 1938.
1938 D-18 #71539 has the rear-shifted X-brace with no popsicle brace
1938 change to rear-shifted X-brace
1939 D-28 #71968 has the rear-shifted X-brace with no popsicle brace
1939 D-18 #72618 and has a 1 & 3/4 nut with no popsicle brace
1939 D-18 #72702 has 1 & 3/4 inch nut WITH popsicle brace. Heel block on this one stamped 23 May 1939
1939 switch to narrow necks (1 & 11/16ths) at serial #72740
1939 The #1 brace, inside near the neck block changes from 5/16ths of an inch wide to 1/2 inch wide, making it roughly twice as wide.
1944 last scalloped brace guitar was approximately #89926. Though some models have been seen after this number with scalloped braces, and before this number with tapered braces.
1945 Either Adirondack Red Spruce or Sitka for tops
1946 Sitka spruce used exclusively on the tops
1947 Last Herringbone D-28 #98233 in 1947
1947 Kluson deluxe tuners appear
1948 Sealed Kluson tuners appear
1953 "magic" spruce? Luthier Dana Bourgeois did an interview with C. F. Martin III in 1984. The interview was in preparation for an article by Eric Schoenberg and Bob Green on the history of the OM model and was published in the March 1985 issue of Guitar Player. Bourgeois was asked to sit in on the interview, and in the last two paragraphs of his recollections especially interesting: "One footnote that I do remember distinctly is that Mr. Martin said that in '52 or '53 the Martin Co. bought a large supply of Engelmann spruce in the form of government surplus of building material. Though he preferred Red Spruce, it was no longer available after the mid-40s because all of the large stands had been decimated. Mr. Martin would have liked to switch from Sitka to Engelmann because he felt that Engelmann was closer to Red Adirondack Spruce than Sitka was. He could not, however, find anyone who was cutting Engelmann commercially, so they went back to Sitka." This nugget of information caught my attention because for many years I Of course, aside from the color of the tops, the anecdote does not in itself prove anything. But it at least suggests how the story might have gotten started.
1958 Style D28 gets Grover rotomatic tuners
1964 (summer) - It's generally accepted that hide glue was replaced with aliphatic resin (titebond) after the move to the new Martin facility. (But hide glue was still used until the mid-seventies for gluing tops and backs.)
1965 Grover rotomatic tuners on all D size models.
1965 Martin switches to short drop-in saddle (On D-18's, likely others as well)
1966 Boltaron binding started. White replaced ivoroid and black replaced tortoise.
1967 Black acetate pickguards first used. Replace tortoise colored celluloid
1967 Grover rotomatic tuners on all models (lower models got "slimline" Grovers).
1968 rosewood bridgeplates on all guitars #235586.
1969 rosewood bridgeplates are larger on "D" guitars #242454 (1988 Longworth pages 57-58)
1969 Brazillian rosewood is replaced by Indian. The first Indian guitars were four D-28's ser# 243644-47. The change in regular production may have started with #254498, though I've also heard that the number varied by model number, possibly as follows:
There have been two documented 1970 Brazilian rosewood D-28's, which are anomalies. The first one was #265783, which was retopped with red spruce in 1993. The second one is serial number 265941. There are several D-35's with mixed woods in the 3-piece back. Like D-35 #258962, which has a Brazilian rosewood center wedge in the back. Note some post-1969 Martins have some Brazilian rosewood, in the bridge, bridgeplate, fingerboard, or peghead veneer.
1972 Starting in 1972 Martin includes cases in the base price of most/all of their guitars. Prior to this they cost extra, and thus you'll see many different cases with Martins before this time. It is possible that the thermoplastic cases were the first ones Martin offered as part of the base price as they were first offered around this time.
1985 Martin finishes phasing in adjustable truss rods.
1988 Maple bridgeplates on all guitars #478093 (This entry from 1988 Longworth pages 57-58)
2000 HD28LSV changed top wood & braces from Adirondack spruce to Sitka spruce after serial number 737277. This happened in May of 2000.
2001 first use of Micarta fingerboards and bridges on standard production 16 Series GT models
2003 Martin converts from glued in to drop in long saddles. Happens before serail # 942xxx Glued in saddles have square/straight ends, dropin have rounded.
2003 Martin starts producing guitar necks with "wings". Basically The pegheads have wood splices on the portions that stick out. This is not easily visible from the front due to the peghead veneer. As with many other changes, this one is made to preserve dwindling wood supplies. In this case Mahogany.
2005 Martin switches to bone nuts and saddles on many models. 2005 Martin starts using Spanish Ceder in many necks rather then Mahogany.