@cyberjunkie & @fwcetus,
I found the Soligor 100's optical layout in a Soligor C/D brochure. It's below along with Vivitar Series 1 90mm f/2.5 specs for comparison:
When focusing, the Vivitar's rear element is fixed while the Soligor's elements all move as a unit. So they're completely different designs. But both are a pleasure to use!
P.S. Back in post #13, I included a scan of some Soligor lens specs. It says there the filter size for the 100mm macro is 67mm. Unless there were two versions of the lens, I believe that's incorrect. My 100 has a 62mm filter thread and the specs in the scan above also state 62mm.