Making sense of the history of this processor was what started me on quite a research project today. It really looks like it became the IOP when the system was upgraded to F and if it was part of the system at retirement, that system was never upgraded to Access. It could have originally been a TSB A, but perhaps more likely started as the Main processor of a TSB B. It could have started life as the main of a C but more likely an A or B than a C. I just seems to me to be too big to have started as an A, and too old to have started as a C. If it were to have been a TSB B Main processor, it would not make much sense to purchase a newer processor to upgrade the system to a C (I don?t think C needed the floating point instructions). But then an upgrade from B, C or C? to F would have purchased a new CPU for use as the main (because F needed the floating point micro-code in the main), repurposed the old main to become the new IOP, and retired the old IOP. An upgrade to Access would have retired this processor before the system was retired and the IO cards would have been repurposed in yet another new processor (Access needed both floating point micro-code in the main and IOP micro-code in the IOP.) So, it ended it?s life as an IOP of a C? or F (most likely F). It started it?s life as an A or the Main of a B or C (most likely the Main of a B). All just my best guess from a day?s research. YMMV If anyone is going to ask the seller a question, it would be cool to find out where the system was once installed. Thanks. Hope this helps, Mike Gemeny