Thanks for solving the why:
You shared a five-digit number on barrel vs four-digit on receiver.
Guess the barrel was replaced.
Hopefully someone here will ID that stamping.
Romano, Romano and Romano for the possible work.
He has worked on more original Spencers than any contemporary gunsmith.
His shop is large and VERY well equipped. He can make or do anything you want.
If anyone can fit the two parts correctly, Larry can.
Don't be impatient and risk a goof (stuck half-way mess) by trying LOCTITE yourself.
Like everything, it's the "ya just gotta" aspects of seemingly simple things that get us half-baked mechanics into more difficulties. I have seen LOCTITE work, but you have clearly miss-matched parts, not just barely loose ones!
There are different versions of LOCTITE, which experienced people like Larry fully understand, and headspace to resolve. He does/can!
He's in a head down, home stretch run to meet gun work/delivery deadlines for the second week of May at Fort Shenandoah and unlikely to answer or quickly respond to calls or texts.
Most of us have learned that April & May and Sept & Oct are his busiest times.
Email him at Romano Rifle Company with a full description. He will respond.
Please let us know as things progress. Thanks.
El Supremo/Kevin Tinny