Agree with what Mark said above. When the key is inserted into the barrel, the raised "teeth" of the key retract the pins/wards of the lock. This should lead to a smooth barrel, which can easily move in the cylinder and actuate the unlock/start function. Using a worn out key won't fully retract the pins, and lead to scoring of the aluminum ignition cylinder. If you are lucky to have a decent cylinder, it seems that using a good key would help maintain it. This would likely require an aftermarket key, as they hold up better than the soft brass of the RM/RL/ST keys. The FS keys seem to be plated steel, so they hold up better, but often rust. So an aftermarket key from Romac, Ilco, Dominion, Lucas, or WBH might be what you use for function, and save the original for the concourse.
Aftermarket keys often had a blank side, or largely blank with only the key number. Original RM/RL/ST keys usually had Rootes, L&F, +/- Pentrastar on one side, and usually "Made in England" on the other side. Of course there are variations over the years.
The FS keys were usually the 6 sided "polybow" head, with Wilmot Breeden and the key number on one side, and Union and Made in England on the other. I'm not sure if the round headed FS keys were original, perhaps others can chime in. A confusing aspect is that aparently Wilmot Breeden sold aftermarket keys for a number of years prior to their buy out.
Regarding Romac, it seems that they would sell a whole board of say RM keys in all number combinations. presumably to locksmiths. If you knew yoiur number, you could just go and get a new key off the rack. Not the most secure system.
David