I have the newest version of the Olympia Cremina and purchased a RB tamper to suit, 49mm, but it appears a bit small by at least 1 to 2 mm for it leaves a ring of grounds on the inside perimeter of the basket after tamping. Does anyone know if there's been a slight change in basket size or if I've mistaken the correct tamper size? I checked my tamper and it's an exact 49mm, which, from all 1 could find through Google, should be a perfect fit. I don't have a caliper, but a tape measure shows my single and double basket have a 51 mm inside rim diameter. The double basket bevels inward only slightly so that when the tamper sits in the bottom of the basket (basket is empty) the tamper touches all sides.

I use a Reg Barber tamper with a 49.5 mm base with my '84 Cremina. It works for both the stock Cremina double basket and the MCaL double, which is slightly deeper. Both are in a Penney naked PF (but they also work in the stock PF).

I also own a new Cremina and a RB 49mm tamper but it fits pretty good. Maybe a 0,5mm of room left but not much more. I indeed have to tamp then take the tamper out and remove some grounds and then polish. It isn't a snug fit but fits well enough. Maybe 49,5mm should be perfect but probably not much bigger than that.

Probably not, but I would strongly recommend measuring the basket diameter with digital calipers. That's how I came up with 49.5 mm for my two MCaL baskets and the Cremina.

Which clearly means there is variation in basket diameters so correctly measuring the basket you have in your hand is essential. I should add that my RB tamper base isn't flat, but the C-flat profile.

Yes, of course. But I understood the thread to be about sizing the tamper to the basket. The OP was concerned that an off-the-shelf 49mm tamper was too small. If the OP wanted a tighter fit, it seems the only way to accomplish that with any accuracy is to measure and then go to a supplier that can produce or offers as stock the required size. These aren't VST precision baskets after all and some minute variation in diameter (or depth) is to be expected. I'm not sure why measuring something to assure proper fit is controversial.