As you say it is an older trailer. You may find you need to do more then just the seal. As you only need to move it a few miles every year, consider this. Sand down the spindle as best you can, pack the bearings, replace the seal, install bearing buddies, pump it full of grease, before going to the launch site, at the launch site, after launching the boat, before retrieving the boat and after retrieving the boat. Keep that seal covered in waterproof grease, don't be concerned about the seal leaking grease at the inner seal.
If you do find out which speedie sleeve you need you will likely need to get a machinist to fabricate an installation tool as they come with an installation tool that is too short. I have heard that you can use a piece of PVC pipe but that depends on the speedi sleeve dimensions. you might get lucky, Oh and consider ordering two.
I have installed speedie sleeves on large seal surfaces by heating the sleeve in boiling water to expand it and the slipping it over the seal area. This is a one shot deal though, if it shrinks onto the seal area before you want it to, you are sol.