I have just torn down the worst example of mechanic's build quality I have ever seen. Having pulled apart dozens of M20s as well as S & M 50s and 52s, never have I seen such bad work. The head was shaved to 139.3mm (WELL past the 139.7 service limit), and it was bolted down using a 3mm think steel sandwich head gasket. That was NOT a misprint !!! The head had JUST been "rebuilt" and most of the valves were leaking, leading to leakdown numbers well above 50%, and compression figures from 30psi to 145psi. IT GET WORSE --- Five rods had F5 end caps, one was a D5 and faced the opposite direction of the F5s, all were backwards on their respective rods, and none of the stamped numbers matched (caps to rods).

So, my questions are;
Specifically what do the letter/number marks on the caps mean?
Are the serial numbers on the sides of the caps and rods (at the big end) always matching?