Quoted from wild_pins:
Small update, I've been waiting for new 5101. This time I thought I'd try from my local electronics store.. took about a week to get them in the shop... picked them up today and they were branded "PCD 5101AP".. Other markings are "MRM8231 2 Y" and at the bottom "21655".
Can't find a datasheet, though.. and some sites refer to the manufacturer as "Sig", though no manufacturer is on the chip .. will give it a go tomorrow and see what it does, but my hopes are not that high.
that is a good part number. The A suffix is probably insignificant, I don't see anything in the datasheet about it.