I have two Canon printers, both Uk models:
TS 5050 (A4 scanner/printer) uses 551 ink cartridges.
iX 6800 (A3 printer only) uses 571 ink cartridges.
I have always resisted the temptation to purchase third party (often much cheaper) compatible ink cartridges.
The colour ink cartridges are by chance, for all intents and purposes identical in shape size and fit in the printers (as are the PGBK 550 and 570 black cartridges), only the cartridge type number seeming to be the only difference. Other that when fitted in the "wrong" printers that the red light fails to come on does the difference become known, meaning the only other difference is the printed circuit board on the cartridge are also different. I know as I "accidentally" did this recently.
I must say straight away that I was not aware of the cartridge phyisical similarities when the printers were purchased and was not a factor in the purchase, but since they are...
Is there any software/firmware update that will allow these cartridges to be swapped between the printers, apart from anything else it would make keeping stocks of cartridges so much easier?.