http://www.pcmcia.org/pccard.htm#lowv
It appears the amiga slot is keyed to accept 5V cards according to the above link, and that someone has to file off part of their 3.3V card to fit it in. However the card would have worked fine, as the amiga is indeed 3.3V!
The low voltage (3.3V) key on the card is thicker (2/3 thickness) than the 5V key (1/3 thickness).
It appears that the PCMCIA specification also allows another low voltage operation of X.X, but that must not have been used (much if at all).
As to cards (and slots) that are supposed to operate at both 5V and 3.3V, not sure what they should be keyed like. (I am guessing the card would be keyed like a 5V so it can fit in all slots, and the slot would be keyed like a 3.3V so it can take all cards, but you never know).
Here's another link showing the thinner edge of the 5V pcmcia card:
http://www.lindy.co.uk/tips/PCMCIA.html