Thanks to Glenn and Mike for their pointers. We've done some more analysis,
and can now report:

The 16-bit value is, apparently working properly:

16bit    Hex(16 bit)    Spoofle#    Hex(spoofle#)   32bit  Hex(32bit)
-32287   81E1           481         1E1             963    3C3
-32286   81E2           482         1E2             965    3C5
-32285   81E3           483         1E3             967    3C7

So, a transcrition error on my part for the 16 bit value. Apologies!
However, the 32 bit result
seems to have suffered a spurious rotation (rather than a shift?) to get its
value, because the value is always 2*spoolfilenumber + 1, which is the
result you'd get from a rotate left....

I'm certain this is a feature, not a bug, but I can't for the life of me
imagine what I'd use it for!
So, I'm gonna hope that there are never any spoolfile numbers > 32767 on my
systems, and use the
16 bit value.....
Tony