It should be possible to use any later Commando frame, and the '69-'70 type that has the support tube should be a direct replacement.
Changes were made to the 1971 frames that could be worth noting, as 1971-on frames lost the centre stand pivot cross tube from below the gearbox, a cross tube being fitted '71-on, further forward (with no centre stand brackets) as the centre stand pivoted on the engine cradle from that point on. And I believe the side stand mounting also changed again?
The head bearing assembly changed to a non-adjustable sealed ball bearing type, and the yokes (trees) were also changed, the stem becoming part of the upper yoke with the securing nut beneath the lower yoke. Your early yokes could possibly fit the later bearings without much (or any?) modification, and others here may be able to answer that?
1972 frame side stand mounting changed again I think?
And, as kommando has already mentioned, the 850 frames have a (1 degree) difference in the steering head angle to 750 frames, so the matching 850 'ANG'-marked yokes should really be used with the 850 frames. But apparently it is possible to use the 750 yokes with the 850 frame (according to information given in the NOC Service Notes)?
Apart from that, there's no significant differences between 71-on 750 frames and the 850 frames that I can think of? Although the 850 Mk3 had a hinged, lockable seat, so a small hinge mounting bracket was added to the L/H frame seat loop along with some minor changes to both inner shock mounting plates, but those differences should not cause any problems.