Ordinary Japanese pens of the fifties and sixties are not well covered either in the various pen books I have or on the web. Searching for Pilots, I found plenty of references to the Capless/Vanishing Point, the Myu and the Murex but little else.
The seller described this pen as the Pilot Super 150V. I believe it was made from 1956 on. Some were made in the Pilot plant in Brazil. I don’t know whether this pen was made there but the inscription on the nib says that it was made in Japan.
Looking at this pen, I see hints of other things. The barrel shape and end remind me of the Parker 61 and the nib is like the “fingernail” nibs that Sheaffer made around that time. The anodised cap looks like those on Pilot’s long-shorts. I think these resemblances are no more than Pilot following the fountain pen fashions of the time.
I like fine and extra-fine nibs and that is where Japanese pens, Pilot especially, really shine. This pen is an EF and a particularly good one. It’s a cartridge/converter filler and unlike some other companies, Pilot has never changed the style of its carts so the present day Pilot cartridges and converters fit.
I think it is the very unfamiliarity of the older Japanese pens that appeals to me. I have occasionally had prewar Japanese pens, ink-in-the-barrel pens with a shut-off valve and sometimes exceptionally flexible steel nibs. After the war, as illustrated by this pen, Japanese manufacturers quickly caught up with more modern styles and filling systems.
Goodwriterspens's Blog 
Andy Lambrou and Masa Sunami’s book ‘Fountain Pens of Japan’ has excellent coverage of all the major and minor Japanese makers. The Pilot chapter doesn’t show precisely your pen, but it does show a Super 150 with a transparent ‘demonstrator’ section and a Super 200, which is also very similar to yours. I understand Super 100, 150, 200 and 300 referred to similarly-styled pens of increasing diameter, date of manufacture from c1956 to at least 1962 (according to the dated pictures in the book).
‘Fountain Pens of Japan’ is eye-wateringly expensive, but it is almost certainly the most beautiful book about fountain pens that has ever been produced. Given the price, you might expect it to only feature the high end pens, but to its credit, plenty of the more mundane pens are also pictured. I took a chance on buying one at a rather more reasonable price on the Amazon second-hand market, and I was delighted to find the copy I received was signed and still in mint condition.
I was aware of that book and went searching a few days ago. The cheapest I could find was £90. Considering how seldom I am able to find vintage Japanese pens that I want, I couldn’t justify laying out that much, despite being sorely tempted. Anyway, I’ve used up my pen book budget for the moment. 🙂 In the unlikely event that I find myself flush with cash I will go looking for the book again.