What a wonderful post and watch, Nicolas!
HUGE congrats, my friend!
This is a theme we (and many other Purists) love and share!
As for the Cal 501 vs Cal 552 - as you know I am not a Omega expert, but some of the differances I suspect (based on that the 552 is the same base caliber as 551):
The diameter is not the same - the 501 is 28.50mm while the 552 is 28.40mm.
The bridges - 501 has separate brigde for barrel and center wheel while the 552 has combined barrel and centerwheel bridge.
In total the 501 has 7 bridges while the 552 has 6 bridges.
The 501 use 14 jewels for the gear-train while the 552 use 19...
But as I said - I am far from an expert.
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As always - such a wonderful post, my friend!
Here is a later sister (2913-7) saying hello!

Best
Blomman
PS. ...you have a very clever friend!