[Milsurplus] Stain Removal Help

[J Forster]

Has anyone tried to remove coffee (or possibly coke or tea) stains from
the covers of TMs? I have several vintage original TMs that have what
appears to be coffee cup rings on their covers. The covers are the usual
yellow index card stock. I'd like to get them off, but if it's a
destructive process, they'll have to stay. A web search turns up lots of
carpet and fabric links, but nothing that seems useful.

Thanks,
-John