There is hardware for running an HP signature in the 9010a. I found this in the service manual (page 4 of 4 of the main board schematic, or something like that, there was no page number). Included in the calculation was a counter that counts the number of high to low transitions made by the incoming data bitstream.
Is this the Async clock module? (where you looking at the PDF from spies or do yo have a schematic?)
If this circuit is being used to calculate the sigs, it could be that this count value is being added back into the calculation to convolute things. If this is the case, the above solution won't work.
This could be tested by, finding two identical signatures, using different bytes, that created the same CRC, and then adding a third byte to see if they are both still equal.
2 identical signatures for the 9010A? we've got a collection of a paultry few. I did run across one the otherday but I'm pretty confident it was a typo. Alex Yeckley are you reading? Kev