I came across K152 on reverb in Italy. The seller was kind enough to put up the K, chassis date and guts. I can't make out the RPxx on the pcb but the tracings look like my SP9A's on the V1 V2 side of the board. They do not look like my RP 8C or SP 8B coli's.
Between 8/18 and 8/26/81 there is a gap of about 25 K's, not that all of those K's have dates in between. It is possible they had mostly built amps waiting for a component and pumped out a bunch when the part(s) came in.
Seeing what must be a RP 9A in K152 means between K123 and K152 there was a change from RP/SP 8B to 8C to RP/SP 9A in the span of roughly 30 K's. Over time we'll get this narrowed down even more.
It seems like the 8C and the 9B might be the rare birds of the coli run and I still have yet to see the 9B. And I'm just guessing that the 1st coli's had the RP/SP 8A which I also haven't seen. If for the most part there are no pre-K100 then the 8B boards are also rare in a coli.