OK, here's what I believe.
Your engine, frame, and certification label left the factor as they are.
1) By June 1975 the last six numbers stamped into the frame matched the serial number on the red certification label and the entire frame and engine numbers matched.
2) Your certification label has almost certainly always been on the bike because it has the original type and material hammer drive screws.
3) If the top of your gearbox has the same last six numbers as the other locations, then at least the gearbox shell is original.
4) It is not a "76 MK3". If it's titled that way, it's no big deal. In most states and probably Canada as well, the title/registration were essentially done by the dealer and since Norton had no "model years", they usually put the date when they did the work. You bike clearly states 6/75. In truth there's little clear about even that. When the records exist for 850s, there are about 6 data items and those tell you little. 750s usually have much less than that!
The other things interesting to me are:
1) What head (should have RH4 or RH10 stamped into the head on top near the timing side exhaust rocker.
2) The thing I wish all would tell me is the number like 00xxx which is sometimes stamped into the drive side of the head stock and knowing there is no number is just as important as knowing what the number is when is exists.
3) If there is dispatch record, I like to include that data.
4) Anything else you want to provide.