Hi,
I'm only a jewellery buyer but afaik a hallmark is the multiple marks made
by a uk assay office.
If you want to provide an honest description just say they are "stamped 375"
which you believe means they are 9K gold.

Do make sure you're know they're gold. I bought a "9k" necklace from ebay
recently which only had markings on the lobster clasp. These clasps can be
picked up for a few pounds from the www. There were no other markings on
the chain itself. The clasp was newer than the chain and the chain plating
was badly worn so that you could see pitted brass coloured metal beneath.
No way was it solid. I sent it back.
Other chains I own usually have markings on the first chain/jump ring to
indicate their karat content.