This is for an airsoft model and will literally not work on the real thing, which has a totally different mounting system, don't waste your time.
This is a quick and dirty buttstock rail adapter for the KWA/KSC VZ61 GBB/GBBR Airsoft Toy.
This was done fairly quickly, and the fitment is OK - its nice and tight when printed in PLA, but for field use, I would recommend at LEAST PETG and ideally PA-CF or another nylon.
The author marked this model as their own original creation.