Here's a page from Mina Colvin's catalog (Colvin gardens) from 1979. What I liked about her catalog is that she added personalized descriptions of the plants she sold, I had bought a fair number of semps from her. The list of cultivars was long, almost 10 pages, on page 8 we see 'Rubikon' (although spelled Rubicon), 'Rubicon Improved' (also not using the "k" spelling, so I wonder about this "k" spelling thing), and 'Rubicundum'. I would pour over the various semp catalogs and make notes.
I find these old catalogs give useful insight on cultivar ID, whether they're correct or not. In 1979, prices for most semps was $1.00, a few years earlier many sold for 70 cents, LOL.