The 55 lumen Niteize LED works pretty well in the MX991, it's a floody light, so higher lumen drop-ins don't look all that much brighter. The ~2-9 Volt operating range of the NiteIze drop-in makes it easy to run on different cells – with adapters I can run the MX991 on 11 types of cells, including 9 Volt batteries.
Originally these came with 3 red filters, but they replaced one of the red filters with a blue one beginning in 1985. The white one is the black out filter used for blacked out vehicle convoys for signaling at short distances. The clear filter is a diffuser.
I only have Fultons, so I'm not sure if the gaskets are the same on the GT Price, but I'm guessing that they are.