My understanding was that they were the first style of receiver and are prominent in 1943 and early 1944 'a' and 'b' suffix blocks. They then disappear for awhile but they do show up in 1944 (especially starting in the 'p' block) and again throughout 1945 but these would likely be previously rejected receivers that were forced into production. Some evidence of this is many are generally all G43 not K43 marked, where as milled receivers that fall in the same block are mostly K43 marked.
Claus has a good serial number study and Tiger Tank could likely add more but seems to be MIA.
https://www.yumpu.com/en/document/read/17370557/g43-k43-serial-number-studies
They are generally WaA marked 359, so I believe they would be 'ac' production as they don't have any other WaA marking on the receiver, unlike the cosmetic receivers which were contracted.
I think the panel cut was eliminated as it was simply an unnecessary step in production that could be eliminated.
Out of the 10x rifles that I have decided to keep, this was the last variation that I added to my own collection. Realistically, all I need is a 1943 and I should have a good representation or production variations.