Full disclaimer.  I am the third owner of this item as far as I know. 

It is a G&P underslung Long M203 grenade launcher, complete with accurate markings designed to fit on Tokyo Marui M16's

https://www.fire-support.co.uk/product/gp-military-m203-grenade-launcher-for-marui-long

The gun I was intending to use it for is a G&P M16, of which it did fit, but not secure enough for me to be happy that it wouldn't twist in game play or potentially damage the rif.  There is a nub on the receiver end of the M203 made to fit G&P M16's designed to stop this twist.

The launcher works fine, having tested it on a nuprol 72 bb shower shell (I was equally amazed by it being still gas tight).  The safety lever also works.

The launcher does has signs of use and the first owner (the seller denied all responsibility for these quirks) dremeled a section of metal off on the receiver end making that right angle cut as seen in the photos.  I have no idea why it was done, though it doesnt affect its mounting. Maybe they were trying to reduce weight?  The other oddity is that the metal front end of the heat shield is missing, also shown in the photos.  No idea why it was removed, but it did stay on my gun, admittantly with a little play.

My intention was to be able to put on and take off the launcher depending on if I fancied using that day. As a result I dont want to fettle a solution to making the launcher fit more securely, as that would make such swaps harder to manage, especially on a make of gun it was not originally designed.

Looking for £65 all in inc postage, collection considered and personal delivery possible depending on distance and fuel money.