The fact that the PCB part number in an HP product has a particular rev level almost never has anything to do with the suffix of the model number (e.g., 9915A or 9915B). For instance, the HP-95C and HP-95LX, which aren't even *similar* products, have 00095-xxxxx parts, with various PCB revs, as does the HP-9807A Integral PC. (Maybe the Integral was originally going to be designated an HP-95 with some other letter suffix?) It's probable that the 9915A and 9915B have different PCB part numbers, though both would likely be of the form 09995-6xxxx. This is an HP 5x5 part number; the first five digits are usually (but not always) the base of the model number. PCBs are usually a -60xxx or -66xxx suffix; repair exchange boards are usually ordered with a -69xxx suffix, though the board etch will show the original part number. Subassemblies usually have -8xxxx numbers, and documentation -9xxxx. Sometimes on a 5x5 the leading zero is not shown, but it's really part of the full part number. 4x4 part numbers are for "generic" parts, that are not specific to a product. Things like resistors, common ICs, and such. Some 4x4 numbers may only be used in a single product, though. In the 60s, and maybe in the early 70s, HP had 4x4 and 4x5 part numbers for model-specific stuff, but they all got changed into 5x5 part numbers by prepending leading zeros.