after I did a little research, I found the Corelle Corner, which offers all the history of Pyrex. Here is what I found...
These were actually the last pieces made in the 40s, which makes this the 5th oldest Pyrex & the oldest one in my collection!
The other oldest include:
Remember how I mentioned the T.M. Reg? I thought that could mean something, but it really doesn't mean much as it relates to this particular piece. I did find out that the "older set of the primary bowls because the sets from the 1940s were not numbered on the bottom. Since your set does not have numbers on the bottom look for the markings T.M. REG. in an arch above PYREX and U.S. PAT. OFF below in an upward facing arch. The later sets from the 1950s – 1970s do have numbers." This is where T.M. REG. is more important. However, it is fun to notice some of my pieces from the years after the 40s do not even have T.M. REG. on them.
Also, while researching I came across some great advertisements. Specifically, the "hostess" collection was advertised in the LIFE Magazine September 19, 1949 issue:
& here is a better photo of the advertisement & another fabulous ad I found:
Can you believe those prices? o.m.g. love it!