OK, I have just checked the three spare blocks I've got (one is a boat anchor due to bore damage) and all have a number stamped near the water pump. However, I can find no evidence of any number on the main caps except for the forging number eg:12G207. Am I missing something?

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1964 998 Cooper , 1968 Morris 1100

Ok, Check Near the Hole for main bearing bolt, sometimes it is close to the stepped part of the Cap, If You have lock washers on the caps You may not see them.
also if the bolt is in place you may not see them
I just checked about 30 different caps and they all have numbers on them

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" A Hundred Years From Now It will not Matter what my Bank Acount Was,
The Sort Of House I Lived in, Or The Kind Of Car I drove ... But The World May Be A Better Place
Because I was Once Important In The Life Of A Child"

Now thats dedication John

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"Show me the Mini!"

I must say it was not that hard, Over the years I have saved a lot of main bearing caps from blocks that where otherwise stuffed
from 998/1098 and 1275,
I am not saying that there are not main bearing caps without Numbers, I just have not seen any

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" A Hundred Years From Now It will not Matter what my Bank Acount Was,
The Sort Of House I Lived in, Or The Kind Of Car I drove ... But The World May Be A Better Place
Because I was Once Important In The Life Of A Child"

I`ve seen a few sets of main-caps without numbers over the years,,, maybe 3 sets tops tho

now i`m just guessing but i`d estimate that the ones without numbers would (or may have been) replacement caps ... just a guess tho

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No offence intended here but--> anyone writing a book about minis 30 years ago may not have experienced such worn or stuffed-with components as we are finding these days.

You should put your heart & soul into everything you do.