I bought one off of Ebay, a Hong Kong based seller with 100% feedback rating. $370 total with shipping.
I suspect this lens is an asymmetric Sonnar type design, like the Zunow 5cm F1.1 with some of the "filler" elements left out, like the C-Sonnar- technically, an Ernostar. The Sonnar is the Ernostar with low-index of refraction glass used to fill the space of the 2nd and 3rd elements to reduce air/glass surfaces, which increased transmission. The advent of multi-coatings basically got rid of the 4% per air/glass reflection, and allowed the elimination of the filler. This 7Artisans lens- well, I'll do a write-up after I get it in hand. I have most of the Sonnars to compare it with. The 50/1.1 Sonnetar is a 5 element in 4 group lens- basically the 5cm F2 Sonnar without the filler of the front triplet. This lens is 1/3rd the price of the Sonnetar. I have the 50/1.1 Nokton, 50/0.95 Canon, 50/1.2 Nikkor Ais, and 55/1.2 Nikkor Ai for Super-Speed 50s. I've had a couple of 50/1.2 Canons in LTM, sold them.