My '72 CB500 has the number stamped right on the face of the lock. Just flip the cover aside and the number should be there. The number on mine is T9927. I purchased three replacement keys from a guy on eBay. They sell them by the number as new or used old stock pre-cut keys.
The Ignition, seat lock and fork lock all use the same key and the number is stamped on the face of all three locks.
MJ