It's a Jacobs-type (not genuine Jacobs) and has no number.

I'll just co-miserate here. I bought a small drill chuck with 40 taper
holder. No numbers. Bought two of the four way keys and still didn't get a
fit. Ended up turning the closest key down so it fit the pilot hole. There
must be a dozen standard sizes and then a zillion non-standard.

Karl