I suspect your guess is correvt, and I suspect they predate 1918 or so. C-Clamps were one of JHW's earliest products, and they had many patterns, however, it is very hard to find pictures of certain ones. The widely advertised standard patterns have slide handles , or a hex head lead screw. The ribbed reinforcment is also not shown anywhere that I have seen (Tho armstrong used it on some of their clamps) I also don't know what the strap clamps look like...
as of 1921, there were (advertised)
4 patterns of regular C-clamps (which later seem to be heavy,medium,light, and ???)
6 patterns of parralell jaw machinists clamps
?? patterns of strap clamps
The Diamond W logo is fairly consistant by the early 20's, but doesn't seem to appear before that..
So a plain W is not unreasonable....
1914: