Just in case useful, my cb500/4 was claimed to be an Italian import and had a different steering lock with a metal cover held on by a brass stud and I replaced the head stock with a UK one so I could replicate my old bike setup and bought all 3 locks/ignition new, with 2 new keys.
After seeing your post I came across the eBay ad in the attached pic which seems to come from Italy. So that might explain what the steering lock is about, unless the UK 550 was different again. May be of use if that’s what yours is like and has no key.
On the main key front maybe not impossible to get a couple of blanks and have a go!