Around 25 years ago I purchased a similar hoard with some 35,000
of this type of envelope, all sent to 5-6 different oil companies in Houston Texas, and dated between 1955 and 1966. These were the regular remittance envelopes for credit card payments (slightly larger than the normal No. 6 envelope), and someone apparently had contacts at all the companies and went by them regularly and picked up what must have been several garbage bags or boxes of covers from each one on each visit. There were gaps in the dates that indicated this was done more on a monthly rather than a weekly basis.
The hoard had already been picked over and filtered (probably several times
): most covers with commemoratives were gone (a shame), and there were no covers from the large towns (a good deal).
The remainder was essentially all small Texas towns, and I ended up with close to 2,000 different postmarks, both machines and hand cancels. Fortunately, one of the previous "owners" had sorted all the covers alphabetically by post office, which made my examination and selection relatively painless.
There was a nice variety in markings which showed the transition between the presence of no ZIP Code and ZIP Code in the cancels which occurred towards the end of the date range. It added significantly to my collection of 20th century Texas cancels, and I've had to be VERY careful ever since to not blindly buy covers from Texas between those dates.
The residual covers are still stuck in the back of a closet. Anyone want a Winter project?
The most amusing part, which has nothing do to with philately, is that the individual who sold them to me apparently worked at a funeral home. All the covers were packed in boxes which had originally contained embalming fluid. The questioning look on the face of the driver of the UPS truck who delivered the 13 cartons to my door was priceless.
I've done this on several other occasions also, with different addressee's, different date periods. It's a terrific way to build a detailed reference collection. The main problem is simply managing the volume and documenting the information for the philatelic community.