From Graham Patterson's (
@grahamp here on Photrio) excellent Mamiya TLR System Summary:
"6.4.6 Porrofinder (also known as ‘Porroflex’)
Mirror equivalent of pentaprism. The eyepiece is offset to the left, and the finder overhangs the left side of the camera considerably. Due to the overhang the camera does not hang well from the standard strap lugs.. Apparent magnification with 80mm lens is 0.5x. This item is first listed in the C3 manual, and is joined by the metered version in the C330 manual. It appears to have been superseded by the metered version by the time of the C330s release. This rear view of a porrofinder
View attachment 415686
The Porroflex version is illustrated in some C33 promotional material as having the name ‘Porroflex’ on the lower front. Early versions were manufactured by Nippon Kogaku (Nikon), though whether this was a sub-contract or third-party product is unclear. Porrofinders in the C330 manual have the usual S-M symbol on the upper front"
They are lighter but larger than a pentaprism. IIRC, they provide a laterally reversed image that is right side up.
I use a 65mm lens as my standard lens on my C330.
As I said, an intriguing package.