As per the pinned note in the EE, before I post this to "Used AR Parts" I'll give you a chance at it. It's not your WTB, but might be an alternative. I saw your original post in the Retro forum, and the replies about using a regular FSB tapped for set screws, and thought this would be a good one for that purpose.
I bought a box of AR parts at a flea market and am listing a few of the things that I really don't need, this is one of them. I can't tell you much about it except what the seller told me, or that I can see for myself.
New take off Front Sight Base from an M4 carbine. I didn't think to ask what brand rifle it's from. No scratches, scrapes or scuffs (what look like scuffs are just the reflection from the flash). .750" diameter, with STRAIGHT pins, and sight post/detent assembly, no swivel. Forge mark is a diamond and the number 2.
There's another WTB looking for a FSB so I'll also give him a chance at it, if neither of you takes it by tomorrow I'll post it to the public.