840 but I'm pretty sure it's the same as the 527. I know the large one fits both but not sure actually about the small one fitting the 840 eyepiece but I would think it would. Have to go check that in a few with my 527 ones
I'm kind if partial to the tenebrex /zco ones and my others have those to so they can share ARD and Sunshade if need be. I don't like the thick area where some of the others screw together at the hinge.
Another member here is what I did but I found new ones at europtic also that were in stock. Cstactical is my go to but they only had the objective one in stock