Nice find. Does it have a mint mark on the back? The oldest walker I found was also 1918 but it had a "S" mint mark on the back. I got mine out of a Brinks box. I too have a lot of empty spaces in my walker collection. The next year I have is 1934.
I dunno, I dont polish or try to clean stuff like that. Sometimes I will use a softbristle toothbrush/mild detergent/hot water to gently wash off any surface dirt but I have seen enough antique shows to know that removing the original patina removes most, if not all, numismatic value
Strange thing is the last two boxes from one of my banks have been very skunky, with the exception of one Walker each. Lots of marked coins, no NIFCs, no proofs, and then a Walker.