54 Izzy barrel collar quetion?

[jstin2]

I was oiling my 54 Izzy and noticed markings on collar. Took out my refurbed 54 Izzy and it had similar markings. Both have line up marks and one has a 0 and the other has 8 on receiver and barrel collar. It almost resembles a S/N but does not match receiver or other S/Ns on carbine. Suggestions what it is or means? My heavy refurbished 53 Izzy does not have same.

https://ibb.co/album/nxzSOv

[jstin2]

I noticed a difference between the 2- 54 Izzys. On one the bored hole that houses the trigger tang latch is through the receiver and the other is not. Was it a modification for later production or assembly worker? Added pictures to album. Checked a couple other carbines and they are bored through.