Anyone know the diameter offhand? Hope to find a mercury-based recoil reducer that will fit the factory hole without mods.

Friend of mine just installed one in his NO.1 and can't remember the diameter, but it was the largest one in Brownell's catalog and he did tell me that it took a bit of sanding with a dowel rod inside to get it to fit and it was snug. He placed a short section of dowell rod behind it I understand to secure it in place, but said overall not a big deal installing one.
Believe the O.D was 7/8ths/inchx5", but quick call to Brownell's tech department would help.

Bill I have one in my #1 .416 Rigby. The butt stock hole diameter is 7/8". They work very well. These guys can help you.

http://www.mercuryrecoil.com/

just put one is a Ruger laminated stock. I got the 7/8" model that is 16 oz. I had to remove the shiny label on the tube, and also do some minor sanding in the hole to get it in, but a 3/4" tube would be very loose.

I just chuck a dowel in a cordless drill and tape 100 grit sandpaper around it to ream the hole out slightly.

Thanks guys. The Lott came today, so I can double-check, but sounds like the 7/8 X 5-inch 16-ounce unit is the one to help tame this Tropical.