automatic transmission dipstick "washer"/bushing

Being a relative newbie, I have used the search to find this to no avail...

On my 1988 300E I have lost just yesterday while checking the fluid level the washer or bushing that fits around the red plastic piece right on top of the dipstick. I'm not sure how important this piece is and wondered if I could buy it separately (part number??) or need to buy a whole new dipstick??

Thanks for your help! Mike

P.S. I need to update my profile as I've recently traded my 300D with my uncle for the 300E--the wife did NOT like the diesel :-(

[jgl1]

The rubber seal (# 126 997 00 83, $1.25) prevents dirt from entering the filler tube.