Hi,
If I recall there is a variation of selectric keycaps fitting on beamsprings from early to later ones. Some are very tight and some can be very loose so I would try to find those old threads. Searching google for stuff like "selectric keycaps on beamspring" etc. Try to check that first before ordering any keys.
I think I remember that later beamspring "dark shell/light legend" replacement keys used a different plastic for legends versus earlier production keycaps. So you occasionally find a few keys that have a very strange standout "yellowed" legend. I am pretty sure it is not from smoking, just from someone ordering a new keycap to replace a broken off one.
Honestly, I am not sure what can be done about it. I have never attempted to brighten/whiten any beamspring keycaps. All my keycaps have sublegends on them, and on the light keys they are all silk screened and are easily wiped off so Ive never done beyond a basic clean.