Found this Russian forum on the Neptune.
Страница 1 из 19 - Ружьё Нептун - отправлено в ОРУЖИЕ: Совсем недавно узнал новые цены на изделия тульских мастеров - 60 -ка 12800(не титан) !!! почти три "крысы"... кто купит???
Unfortunately useful information is scattered amongst the usual put-downs and squabbles and derails that threaten to go off-topic. Neptune guns seem to need continual work while other users claim no problems. It appears that there is more than one maker of this gun type and the Neptune is one example, the Galatea is another. Some of the posts can be read as comedy if you are not caught up in arguments and conflicting opinions.
This forum comment caught my eye and made me laugh.
"-----, unfortunately, I didn’t take a picture of my Mironov (a maker, my note) when, after the purchase, I did a complete disassembly and replaced all the O-rings and made an audit. Workmanship made an indelible impression on me - all the details are perfectly processed and polished in a mirror, the springs are of excellent elasticity, it whispered a work of art, it even has a 90° locking prong. It feels like this copy is approaching handmade products. You can easily get acquainted with Mordasov in Hunter's old report, where even in the photo you can see the clumsiness and sleaze of workmanship with the naked eye. Sincerely."
I have used Hanter’s (or Hunter, the auto-translator put in the “a”) photos here and some show the rough nature of the parts found inside some years ago. See below! I would think things have changed, however it confirms my suspicions that these are not mass production guns in the sense that say a Taimen or a Pelengas is.
This is the slot in the tank that the line release finger pokes through! NB tank is pulled forward here.