I could be completely wrong, but this clamp appears to be too small and would not be large enough to hold a 65mm C-band feedhorn.
The unit is adjustable in height, but the size of LNBF clamp is limited to the standard KU size of 40mm and the smaller throat size of 23mm.
I believe you are right. If so, I can always us it anyways and order a C band LNBF older at a later time.