Has the same reference as mine: 8846 N.
That you don't find any numbers in the inside of the case back is not strange.
GP used to put the reference in the inside of the case back up intil end 50' / beginning 60's (not sure about the exact date).
After that they switched and put it between the lugs at six o'clock.
At some point during the 70's GP started to put the reference on the outside of the case back.
All this are my onw conclusions from observing different watches - no hard rules but may be used as a guideline...
Your Olimpico has a wonderful pationa, my friend.
Please don't refrain from showing it here more often!
Best
Blomman