Exellent work!
I believe there are two grip frame types. I remember them as square and round. My Supermags are [correction: round], and my small frame is [correction: square]. Just tell them you've got a DW Mod. 40. They should be able to look it up on a chart.
And yes, the .357 Mags were in models 11,12,14, and 15... All small frame DWs. There are lots of other differences between the small frames and the large/supermag frames. In this particular case, I can tell you that the shroud for my M40V is much too large to fit a small frame M14/15.
You should e-mail Genny at DW. I believe you can get just about any part made from the M14 onward, with the exception of the 375 Supermag barrels. (Yes 375, not 357.)
You may find these helpful:
DW Accessories607-336-1174 or genny@cz-usa.com
p.s. Do you have a .002 gauge for your gun?