I think Allen's are genuine Keihin stockist as far as I know - give them a call to check
I called them to double check. They don't stock jets for the CB550. I
think the code for the pilot jets I need is 99124-076
OVERALL LENGTH = 28 mm
HEAD DIAMETER= 5.5 mm
THREAD DIAMETER = 3.4 mm
The N424-22 jets are close, same length, same head diameter, but an M4 thread.