Tony & Paul,
I've come across
this Celeston spec's page which I feel will help in the discussion.
The bottom of the page has the dimensions of the rear hole in the various Celestron SCT models.
This is all making sense to me now. A year ago, a used a 35mm EP with an eyepiece fitting focal reducer in my C5. The new FOV through the EP was TINY. About half of what the EP delivers neat. Now I understand why.
I haven't had a chance to revisit my C5 with other EP+focal reducer combinations. I'll try to over the next few day, and post my observations.
Here's a link to Russell Optics.