First "ASK" of the new year...

[82Trooper]

First...happy New Year everyone.
I do not contribute much...but I sure do take a lot away from this group. Thank you all for your willingness to share your knowledge. I appreciate it.

New for me for 2025 is a Underwood M1 carbine. I obtained it from the estate of one of my coworker's father who recently passed away. I am happy to be the next caretaker....I took it apart and cleaned it yesterday. I have included a lot of photographs, hoping the group can help me decode/understand exactly what I have.

1. any general observations/specifications?
2. what is the "M" on the magazine release?
3."IO" on the upper hand guard?
4. circled "PI" on the operating slide?
5. "W" on the hammer (Winchester?)
6."ST" on the trigger housing?
7. Multiple letters on the bottom of the trigger housing?
8. circled "RA" on the pistol grip of the butt stock?
(the "14" would be an arms room/inventory number...I believe)
9. The "JL B (R?) on the sling cut out also on the buttstock?

Thanks in advance for any information.