Isn't it nice to have FASA-era materials handy...
Well to answer your question, it's always been listed as a Lt Torso mount for the SRM-4. While the fluff lists it in the Lt Arm, the actual TRO entry & the Reinforcements Supplement sheet have it located in the Lt Torso. The same is true of the ON1-M listed in the FASA TRO 3050.
Weird, isn't it?