Hey Rich! I don't know the part no., but I found these spare tires, and curiously I have a bunch of them, in a bag with other parts I have labeled for "Premier Mikado 2-8-2 NKP with PS-2 and Hi-Rail Wheels Item No. 20-3183-1". Hope this helps. I bought that mikado possibly 14 years ago. The tires at first seemed just a bit too wide for the groove in the Legacy Mogul but they seemed to have settled in there beautifully. I also wrote on the bag that they're for a 65" diameter wheel, I must have measured that mikado and came up with that, I did measure the Lionel Mogul and came up with 61" OD. (O scale inches, that is).
Rick