Ok here you go, here is the location of the serial number on the frame of a 4AD, its pretty hard to find under all the usual rust and dirt but its there.

Location of 4AD Serial Number

On this one you see the measuring tape, it sitting on the metal frame support (the metal part welded to the frame, not viisble in this picture) for the body. The location is just approximate but I have found it be to pretty close allways.

Closer detail of SM1500 Serial Location