The Mamiya Sekor E 4/300mm is based on its sibling from the earlier CS series. It's a long telephoto lens with a relatively high f4 speed which, back then, only was surpassed by Canon and Nikon with their 2.8/300mm and 2/300mm lenses. The lens has a pretty unspectacular layout consisting of a triplet master lens in front and a rear achromatic doublet acting as field flattener and short tele converter.

According to the data published by Mamiya it seems to have the same optical formula as the earlier (and more common) Sekor CS 4/300mm. My own tests suggest that the performance indeed is identical. While the Sekor E 4/300mm on 24 MP FF is very sharp (even wide open and in the corners), it has quite distinct lateral CAs - but that's a problem with nearly all "non-ED/AD/ULD/Fluorite" 300mm lenses from that time. Removing it in post processing is quite easy, though. 

At a mere 740 g (measured) the Sekor E 4/300 is probably the lightest lens of its class (the earlier Sekor CS was 785 g). The Nikkor-H Auto 4.5/300 was 1070g, the Canon nFD 4/300 had 950 g and the Minolta MC-X 4.5/300 even 1180 g ... only the Minölta MD 4.5/300 was nearly as lightweight (775 g, but only f4.5 of course).

Despite the lightweight construction, the lens barrel is very well made and entirely out of metal (apart from the aperture ring, but that's really a minor detail). While the Sekor E 4/300mm is pretty large, it feels well balanced on the Mamiya ZM (the ZM has a nice small grip which facilitates handling). On the Mamiya ZE-X, Mamiyas most advanced 35mm SLR, the lens feels nearly as good, especially when the winder is mounted. Only the ZE/ZE-2 bodies are a bit tricky since neither of them has a built in grip.

The biggest disadvantage of the Mamiya Sekor CS and E 4/300mm lenses is their MFD of 5.0 m its MFD. That's even worse than the Minolta MC-X (4.5m) or the Nikkor-H (4.0m). Of course the MD and the nFD are much better (3.0 m) since they are IF designs.

Mamiya Sekor E 4/300mm is an extremely rare lens for sure; mine has SN 10067 and therefore it seems to be the 67th lens manufactured. It's the first time ever I've seen the for sale here in Switzerland, and I was lucky to get it at a reasonable price (CHF 170.--). 

MAMIYA SEKOR E 300mm 1:4                  (5 lenses / 4 elements)

The Sekor E 4/300mm is one of the several Sekor E lenses I tried to buy just after the collapse of the Osawa empire back in 1984 Osawa was Mamiyas main distibutor in the US and elsewhere). The Swiss representative of Mamiya was selling many of the less common Sekor E lenses for a fraction of their original price, and being 15 yo their offers were quite tempting. While the E 2.8/28, the EF 1.4/50, and the E 4/200 were still available when I heard about it, they had sold their last E 4/300 just one day ago. Of course I was deeply disappointed, but alas ... here we go. Nearly fourty years later the lens has arrived now, finally! It might well have been that very lens I missed out on so many years ago ...