RE: bought loose 9mm ammo from a guy at a flea market
Go with a "Dutch mix" and thoroughly see how well your SW feeds. It's only 100 rounds and 5 bucks. It will be gone in 5 or 10 minutes anyway and any info you get will really be useless. It is older ammo of unknown bullet weights/loads so you aren't going to be able to go out and buy anything exactly like something you think you like.