Beware of persons selling overpriced fuel tank emblems for the Yamaha XS 1100.  Often these are inexpensive (~$5) golf cart emblems.  They are typically silver and may come in genuine yamaha sealed bags and have 2 screw holes and may fit the tank but are not the true motorcycle emblems.  Contact the seller and ask for the part number and compare it with the real part number.

As of this writing, real yamaha emblems are available for an 1981 model from an internet parts distributor but are still priced at less than $10.  Some true NOS emblems are still seen occasionally on ebay but BUYER BEWARE.