If you have the tap and it is apart it should be pretty simple to run a vernier caliper over the O ring slots to get the size, or just take it along to an auto parts place and trial fit some from a kit. I have one of those taps here, if I get a chance to pull it apart I'll take some measurements for you.
Okay, the smaller O ring measures around 9mm OD, 6mm ID and 1.5mm wide, the other wider one is a square section rather than an a round O ring having the same ID/OD but being 2mm wide. All measurements are as close as I can tell given this tap is old and they are probably worn and hard.
