If that's a 300mm f:4.5 Carl Zeiss Jena Tessar like I think it is, that camera is a 13x18cm one. The lens front glass is 67mm across; the back looks to be a lot more than 2x that. Same goes for any other 300/4.5, by the way.
Looks like there's a reduction back with it too - you see it in the background in one of the pictures.
Apart from that, I know nothing about Mentors. But it looks to be a good, strong, solid camera - on the heavy side for outdoor work, though.