Belows figures are collected from the Internet via Google search, I didn't measure myself and thus the figures might not be accurate, just for further reference:-
C5 - 25mm
C6 - 27mm
C8 - 38mm
C9.25 - 48mm but it's restricted by SCT port size which is ~45mm
So, it's pretty pointless to use 2" visual back with a C5 or C6? However, mechanical advantage with beefier 2" visual back will keep me using one with my C5.
I did this searching originally wanted to give myself a reason to upgrade to a C6, after downgrading from a C8 to a C5, but now, I simply won't.
I shall check by myself to see if my 40mm Pentax SMC XL can deliver more actual field of view than my 20mm Japanese Widescan Type III. If it couldn't my Pentax would be only useful for my tiny Borg 45ED II, but in any case, I won't sell my Pentax since I truely love this eyepiece, very comfortable, extremely high constrast, second to none color fidelity and sharp to edge.