OK there is a
BSC621-2 and a BSC621-2C. I find no listing for a BSC621. The BSC621-2 is a universal lnbf (Ku)
C band is just C band. The BSC621-2C is a Standard lnbf for Ku. If you are receiving NSS806 @ 40.5 than you are using the plate that came with the lnbf. That plate is for circular signals. To get other sats that are linear you would need to remove the plate.
I prefer the universal lnbf's for usage over the Atlantic. Either will work just fine in North America. The sats over the Atlantic have a tendancy to work in the lower ranges which is where the universal works best. So to see SatMex5 just rotate the dish to Satmex5, remove the plate and scan it in. Do you have the lnbf setup as 2 coax lines coming off or a single with the jumper that was supplied when you bought it?