Here's a piece I acquired a year or so ago, thought I'd posted it here, but perhaps not.
It's an unused envelope with a specimen meter postage on it.


Looks to be a Type II (Holme's Specialized Catalogue) Pitney Bowes machine. This is postmarked 1938, whereas the catalogue listed the earliest Type II as 1936, so I'll assume the SPECIMEN aspect of this is the Lux Radio Theatre advert.