Sarco has been around about forever, and is probably the oldest parts house other then Gun Parts Corp.
Their specialty is/was selling surplus guns and parts for them.
Since the overseas supply has dried up what parts they have are mostly from worn out or unsafe guns that were parted out.
They almost always sold well used parts, often rusty. Sometimes they can be cleaned up and are usable.
If they have a part you can't get anywhere else, you just have to hope you get a good one.
In the past they were good about exchanging a part.
Having orders that are incomplete has always been an on again off again problem with most of the parts houses.
Contact them and tell them about the missing parts, and discuss getting an exchange for a part that's just too bad to use.
Note that like most parts houses and surplus parts, what they have is what they have.