I'm inclined to think they were commercially available items as i have one very similar ,also the chain is normally of the type found on pocket watches and with the explosion of wrist watch wearers during ww1 they had to find a use for all the pocket watch chains.
they also come in aluminium as illustrated by the pic of a bracelet that belonged to a Grimsby chum who then went on to become commisioned into the Sherwood Foresters and ended as a pilot in the RAF.
id bracelet.jpg
idbracelet.jpg