It’s a lavatory
That stop was made by American a standard. It’s 3/8” ips.
It may have a washer on the stem you can replace but it may be a ground joint without a washer and may be pitted.
The issue is a lot of times those chromed brass nipples they’re screwed onto get rotten at the threads and will break off when working on them. You t break off later and floood your house.