I have bought these for years, love them, have 3 pair and dump the dirtiest and buy a new pair every year or so.  They USED to be made in China and I ALWAYS had 11.5 M fit perfectly.  the last pair I ordered were 11.5M, same SKU and everything, but they are way too tight and say made in ...  I forget, Dominican Republic I think.  Anyway the size vs stated size is different.  I shall now try 12M's and see.