Nice job, NS. I enjoy measuring things within an inch of their lives and you gave me fresh fodder. Do you think the change in the profile of the inertia block was to keep the weight the same after reducing the rear wall thickness? Confusingly, J.B. Powers in CCNL #172 shows an M2 slide that weighs considerably less than the increased dwell L379. If this weight is representative of M2's, maybe a change in mass didn't matter to the M1icon so much after all?

TR, Figure 58 in the 1953 TM 9-1276 shows the old slide as usable - again with no mention of dwell or internal dimensions.