Welcome mate
With your mate saying he thinks it's a motomag we're obviously talking about the early mongoose frame with the gusset plate up front with the hole in it.
There are 2 models of supergoose with the gusset with the hole with different "identifiers" to seperate them from the standard mongoose frame or "motomag" as your mate called it.
Supergoose 1 : 1979-1981
This will have flat plate rear dropouts (for the rear wheel) without holes in them as per the standard geese of this era but the tell tale is a slightly flattened tube near the rear droupout to prevent chain rub which didn't appear on the standard mongoose frame. Also the vast
majority of supergoose frames in this style will have a "T" stamped before the rest of the serial number.
Supergoose 2 : 1981-1983
This will have rear dropouts with a "step" in them and with holes too as per the standard geese of this eras. The tell tale for a supergoose this time is the rear brake bridge. This will have elongated holes either side of the hole for the brake caliper whereas the standard mongoose will just have the hole for the brake caliper. Early supergoose 2 frames (1981) will have the extra holes in the brake bridge
AND also the flattened tube for chain rub as described for supergoose 1.
Post some pictures up and we can have a look for you.
Drop me a PM if you like