Please would someone who has the Jobling Pyrex book please have a look for me and report back on the dates of the first two Jobling Pyrex marks, together with what evidence for those dates is cited by the author(s).
The first is the simple "JP" mark, not often seen. The two letters were run together so it looks like a "J" with an added bulge on the top right. We have Janet's mother's EPNS mounted toddy glass with this mark acid etched on the bottom.
The second is the "JAJ" over a crown mark, usually with "MADE IN ENGLAND", seen moulded on most Pyrex sold secondhand here in Britain. The basic deco style* "JAJ" element of this is shown in a PG advertisement of 1 September 1933, reprinted in Baker & Crowe — the earliest date I have found so far.
I would also be interested in whether the authors prefer an overlap, a gap, or an instantaneous changeover from one to the other. This last is very popular with tidy-minded glass historians, but I have yet to find concrete evidence ever cited for it, so you always have to treat it with a little suspicion.
* The "JAJ" in the PG ad. has always looked to me more Charles Rennie Mackintosh (A&C or Modernist) than Deco. The version that actually appeared with the crown on the glass was a little milder.