I have here a cable, supplied with a Belkin wireless router, with RJ45s on each end, and only two pairs !
Yes, I have a few short router cables like that, good for 100mbps but no more.
I only became aware of their existence relatively recently when I bought a gigabit switch and connected a PC close to it that I knew had a gigabit ethernet card, only to see it running at 100mbps.
As it worked fine with a longer standard Cat5e patch cable I investigated further and found a number in my collection - all short, oddly coloured and supplied with routers.
However, as noted above, they're not
all labelled as Cat5.
Just looking at one that I think came with a BT unit it says:
"
24AWG 2PAIR VERIFIED CAT. 5e TIA/EIA-568B. 2" on the cable itself.