Filling in a bit more on wristlets up through the 1917 Blue Book (published late 1916 IMO)--all ads below I think previously shown by others. The August ad above seems to be the first national ad mention of a convertible, whether by that name or not. There was a jeweler ad from Feb 1916 (posted in Jeweler ad thread), which shows they were in play.
1912: Gruen goes big on wristlets--first JCK & national ads seen in May. "Wristlets" tm (?) And an Open-Face configuration with no name. Part of the "Gruen Policy." In jeweler ads (see that thread), we learn they are available in silver, gold, and gold-filled.
1914: We finally see some Hunter configurations. W 3 & W 11 show up in February, but in April we get a good look at those and W 48. Here's a clear ad from June.
1915: November 14k Green-Gold wristlet ads make mention--possibly the first--of a gilt dial to match. Here's a copy from December.
1916: Lots new!
- Graduation ads show wristlets with multi-textured gilt-dials, double-recessed and with recessed area for the brand logo.
- Near end of May we see the "Octathin" wristlet.
- August ad posted above in thread
- And in November comes the W839 with Louis XIV platinum dial. I'd like to see one of those!