Vickers 60 steering pump reservoir cap gasket
 

I noticed that there is no gasket on my reservoir cap. It also appears that there are some models without this gasket, but I can't tell what they are. Mine seems to have a band where one should be. It's iron with a black nameplate, from an 85 300CD. My mechanic showed me the fluid level, and that the front seal was leaking, but didn't mention this.

I've seen 'universal gaskets' that are supposed to go on any of the 300s. Otherwise is it just trial and error for the 'thin' and 'thick' ones?

If I put a gasket on one that's not supposed to have this, will bad things happen?

it's unlikely that it will leak from the lid if it's closed well & snug. the front shaft seal is the one that gets worn, since the shaft is spinning in that seal. the front seal is a cheap/easy fix. you just: remove pump, remove pulley (don't lose the woodruff key), poke old seal with skinny screwdriver or ice pick taking care not to gouge seal mating surface then remove seal, lube new seal and tap into place with a seal driver or close-fitting socket, reassemble.