The TM S-system has the same mechbox all the other TM M4's have. I've worked on several, there is no difference. I have not seen any difference in any of the TM M4 series other than the "New Type" and now the Sopmod. The "new Type" has the one peice barrel and reinforced pin retainers in the upper, the Sopmod is a whole new animal.
ASC & RAAT Age Verifier for Red Deer & Area Alberta