Ha! Ha!, you have a 15mm mod what you need is a 15 x 1 Spigot mod.
Sounds daft I know but you must ask for the full title.
I have an A-Tec Maxim mod, as imported by Jacksons, I also have an A-tec imported by Jaegar of this site.
The Maxim from Jackson fits my R93 perfectly and is very effective too I might add, and the A-Tec from Jaegar has just the thread, no spigot, and is used on my other rifle that has thread only.
It sounds as if you only quoted the thread when you ordered yours.