So does the piece at the top come out? Because I have the large threaded piece right at the bottom, then a second smaller threaded section, then a third section smaller again with a thread. It seems like I should have the large thread section, the second middle section threaded and then a hollow smooth cap with an interior thread. I'm definitely missing that top cap.
I looked at mine pretty carefully. It looks like it was all machined as a single piece, however it is possible that the 510 sleeve is press-fit into the bottom. If so, however it is press-fit with a hydraulic press; it certainly doesn't come out easily.
The only thing I can guess is that IF this is a press-fit part, the original owner broke the 510 sleeve off, and replaced it with something else that fit (so it could be used with an ego head). What you have is
not a screw-on adapter, since the smooth sleeve on the outside isn't there. Also the top outer threads (that we have been guessing are an 808 thread) don't look right even for an 808. They look like too coarse a pitch.