I am not really familiar with the very early (1960’s) Mk-3, but they were very short lived.
The 1970’s Mk-3 had the rounded piston cap. The piston head on the Mk-3 is much smaller than the Mk-2 piston.
The numbers of LP ports on both (the Mk-2 and the Mk-3) were the same in the 1970’s. The main body was the same (old part # 102-7). The only difference was the piston and the piston cap.
Here is a diagram for the Mk-2
Here is a diagram for the Mk-3