Yeh, I wondered something like that too.
For what it's worth, I've had a go at summarizing the changes that I've gleaned over the years:
M85 – near identical to M80 Sherpa, but with Matador gear ratios, longer seat & larger tank. One-piece tank/seat unit. Airbox under seat a la Sherpa. Full width 125mm dia front hub , mud-catcher alloy rims, round barrel & head, bolt-on footpegs, 2-piece header & midbox, triangular rear muffler. Single gusset at steering head (on centreline of tubes). Tapered fork-tubes (at top triple clamp). Alloy top triple, steel bottom, with built-in angular offset (a la Sherpa). Some had high-mounted front guard (early?), some low-mounted (late?). Late ones had recessed swingarm pivot (like early M91/2 Sherpa) & weld-on footpegs & the swingarm was slightly diff to match the recessed pivot. Not sure if they used the longer Pursang footpegs or shorter Sherpa pegs.
M99 – 325 version with square barrel & head, 60mm stroke.
M115/116 – extra frame tubes above swingarm pivot (like late M91/92 Sherpa). Frame seemed lighter, perhaps thinner gauge but still mild steel. Top shock mount moved rear-ward (on bracket behind the rear frame tube). New tank-seat unit. Airbox on right side (somewhat like M107 SD Matador) under removable cover. Angled/offset inlet manifold. Conical 140mm front brake, chrome-lined. Stepped gusset at swingarm pivot a la late M91/2 Sherpa. Pursang alloy triples & non-tapered fork-tubes to suit. No angular offset in triples (which is why these & later Alpina’s steer so differently to Sherpa’s) with 20mm less overall offset. Shorter wheelbase. Shoulder-less alloy rims. Dual gusset (folded over) at steering head. New longer seat; slightly new colour scheme. On these models the rear central-downtube was recessed for the swingarm pivot (like the equivalent model Sherpa) which I believe enabled the engine to be mounted about 15mm further back. I’m lead to believe the fork-tubes are longer than Sherpa ones too but not sure when this occurred. 350 engine had 64mm stroke, bigger crank (wider and larger dia), different crankcases to suit & diff stud spacing from earlier 325 engine (if I recall correctly).
M137/138 – much lighter Cro-mo frame, said to be 8 lbs lighter; very thin gauge which fractured everywhere, despite extra gussets/tubes under the airbox. Left side shifter, right side brake, Mk8 pursing rear hub (chrome-lined), one-piece header/mid-box, boomerang/clubfoot rear silencer, sidestand attached to swingarm (instead of frame), new swingarm (20mm longer?) with what appears to be lugs for passenger pegs. Longer wheelbase, more ground clearance. New clutch cover. My 137 engine has square barrel & head & duplex primary drive.
Later models (165/6, 187/8 & 212/3) had completely different frames, swingarms, tanks, shocks etc to the earlier Alpina’s, with rear engine mount bracket hung from swingarm pivot. These are similar frames to the 158/9 Sherpa’s and very different bikes to the early Alpina’s. Forks, triples & wheels look to be about the only things in common with earlier Alpina’s. They were said to have gone back closer to the Sherpa, but maintained the Pursang triples & Matador/Frontera gear ratios. M165 & 187 250 models appear to have the shorter 158 style Sherpa frame, with wheelbase specified as 50”, whereas 166, 188, & 212/213 models have the longer 159 style frame, with wheelbase specified as 51.6”.
Bultaco brains trust, feel free to correct it where required