This horn popped up on eBay, very reasonably priced, and very much caught my attention. And I purchased it.
It had clearly been dropped at some point, and it was all black! But I could make out the engraving USQMC on it and guessed (correctly) that it was a silver plated military instrument from the 1920s.

This is the horn right out of the box. The next photo is after less than a half hour of work. I partially repaired the major bell issues so it would be playable, replaced one valve string, and greased and oiled the horn. The techs in the military must have taken good care of the instrument.

But then there is this curiosity. Check it out:

The serial number is gone. I don’t know if it was filed off when it left the military or there is some darker tale. Or even some funny story. Based on the patina, it was done long ago.
First impressions are that the horn plays pretty well! I look forward to working it over further in the coming months.