I just pulled out my copy of Beretta Pistols by JB Wood, and he has very little information. My guts tell me that either the second trigger variation was late in production, or it was an option when ordering. In the case of the pistols you're looking at, the proof dates and manufacture dates of the frames are very close just because of the frame design change, but if these pistols were all 1972 proofed, they could well have been built on frames from 1968. But maybe that might be the explanation you're looking for. It had the crinkle finish on the barrel counterweight, and the shape of the slide stop lever was supposed to be slightly different, but I've never held one in my hands, so I can't tell you how different it really is. There was a variation on the 76, called the 76S. They could well have had a different set of first one or two digits for different markets, or they might have had different blocks assigned for different variations, and at some point frames that might have at one point been allocated to, say, Model 71 production might have been reallocated to 76 production after the serial had been stamped.

Those frames had been produced for a maximum of two years by the end of 1969 (if we assume that the lever safeties started in January of '68, and that all those crossbolt pistols with 1968 proofs were built on earlier frames), but Beretta can be a bit all over the place with their numbering system. For that time period, yeah, it's interesting.

Beretta shotgun serial number lookup