The 1991 240sx, for example, has 10 different computers(manual vs standard; california vs federal , etc) so make sure the manufacturer number for your old computer is exactly the same as the number on replacement computer. There is compatibility from year to year. ie 1991-1994 have compatible computers in certain instances.
Here is a picture of two different computers with the 9 digit number circled:

Canadians

Your computer is the same as the american. Just go by the number on the computer. I'm from BC and my computer is the non-california version.

1991 240sx or 24 vs 28

"The ECUs used for the 1991-94 240SX are all interchangeable as long
as the transmission type and emissions type are the same."

Jim wolf technologies list says that there are two versions compatible for 1991. For example, the manual/federal version could be A11-B44_G70 or A11-B44_G07. The last two digits are different. A11-B44_G70 is also good for 92-94. the A11-B44_G70 did not work in my 1991 but it may have been defective. Here is a picture of the two computers:

Links

Codes: retrieving and reading(1991-1994)
Codes: retrieving & reading(1995-1998)
Codes: retieving & reading(1989-90 SOHC)
pdf from JWT, shows how to use a 95-96 ecu in a 97:
http://jimwolftechnology.com/w...D.PDF

Modified by vancouverbc at 1:39 AM 9/4/2008