Evinrude gearcase identification

[trombonecooter]

I bought a bass boat a few months ago with a 1986 110 evinrude (E110TLCDF.) I ordered a new water pump kit for it and when it arrived, I noticed that the impeller was not like the one I had just taken off. The original impeller had a slot and was held in place with a woodruff key while the new one should have been held in place with 2 plastic wedges. I asked the parts man at a dealer, and he said that the gear case had apparently been changed. How do I go about identifying the year of the gearcase so I can actually get the correct impeller?