It is a "D" seal, but I don't know the number.
I tried every sealant known to man - the absolutely only thing that worked for me is this
Hard to find, but the only thing that would stick to the rubber. A Redwood Dealer in Durango said it is the only thing he uses. I tried it after failed attempts with Sikasil, Dicor, Proflex, etc. My local Dealer now stocks it for me and hopefully uses it himself. When we did our annual inspection he used some from Lowes and it failed the next day and the seal slid out of the track after pulling the end screws out. It's been 6 months now on the 2300 MHRV and tight as can be.
Some online info will tell you the MHRV 2300 and Proflex are the same as they are both made by Geocel, the are most certainly not, and removing Proflex after it failed caused damage to my paint.