Calling Lamm ML2.1 owners

I replaced my 6AK5's last Saturday. Although I checked them on a Hickok tV7 last December and they tested fine, I recently aqured a Mighty Mite IV tester and the life test/emssions showed they were poor. They had been installed for several years without issue, but there was a substantial improvement after the replacement (i.e. after 45 hours burn in). I have found the V2 6C33C to fail most frequently, followed by the 5651, V1 6C33C and finally 6N6p. 6AK5 is at the end of list as I have rolled the 12AX7 position several times to have any meaningful life expectancy judgement. The good thing about a failed V2 6C33C is that it can be used in the V1 position and vice versa, as the amps only use half the tube. So when I lose a V2 6C33C I just put it in a box for when a V1 goes sometime down the road.

Yesterday I replaced my first tube, a 6C33C-B after 3000h of use. The others still run strong. A pretty good tube design. Btw. when someone is interested to try a good 12AX7 tube without spending a fortune ---> Psvane

I have a question about the 6c33c tubes? The tube store is marking them form 18 - 39 on the box. I wonder what that means and which should I be using for V1 and V2 position to get the best match?
Thanks.