Hmm looks like a boxplate
I don't sell my finds but looking around at similar condition at retail relic shops they want about $100 for them with no attachments and some lead fill issues, that one is missing a lot of the lead so maybe $80 ?
If he sells it to a shop they will offer about half that.
I would google US Cartridge Box Plate and try to find one listed for sale in similar condition.
You should go detect his property. Lol