The earliest Pentax 6X7 was produced in 1969-1971. These cameras have some differences with the next cameras produced in 1971-1976.
So, the earliest cameras have pan head screws (not Philips), round spring loaded locking lugs underneath. Some differences disappiared gradually while production. But there is another notable difference on really earliest cameras: the film type switch was marked as “10/21” shots instead of “120/220” film types on the next cameras. Yes, the earliest Pentax 6X7 was able to make 21 shots on 220 film type. But the film transport was poor, frame space was very unstable and camera continued to produce but with film transport for 20 shots and the switch was marked as “120/220”. Many cameras were returned to Asahi for fixing the problem and modify they film transport for 20 shots.
Another notable difference is the single red dot mark for 120 and 220 films. After some time the film transport was changed due reliability and the single red dot mark was replaced by two separated marks for the 120 and 220 films.
Film plate on early models did not have additional spring roll that makes better film friction with the special rubberized roll that do proper film transport and shots counting.
Many earliest cameras are really old and heavy “worn”.