There ae collectors who specialize in these government departmental "Official Business" (OB) envelopes. They could be collected at either or both Federal and state government and local levels.
I personally collect covers used by Texas State Government Offices, as well as those used at the county OB level. Several years ago I created a series of monographs based on such covers for about 30 states before I was distracted, but those I prepared can be seen on Stamp Smarter at this link
https://stampsmarter.org/learning/S...vCovers.htmlFor Texas, the majority of covers illustrated here are from my collection. There are two monographs, one for the Texas State Departmental covers, either with a simple corner card or the "All-over' style, and the second is for various county offices. Many of these county offices purchased "All-over" decorative envelopes from Clark & Coates Stationers, a firm located in Galveston, which are both attractive and monotonous, but fun to watch for. They changed frequently at the county level, as it appears each new elected official would order his own personalized envelopes, so one can find multiple variations of the envelope and corner for a series of individuals in a particular office. This Texas group in these monograph are now only a small portion of my collection, which grows frequently.
The other states on this Stamp Smarter page are essentially "pseudo-monographs" based on images which I harvested from
ebay and occasional auctions. My intention was to provide potential collectors with a starting point showing what might be available. While I continue to "harvest" additional images to expand the monograph for the states (both present and not represented), I suspect that my life-time will not be long enough to really do anything else with these online.
That said, anyone who wants to do something like this and would like to get a quick start with identification of what I have seen but not posted, contact me and I'll provide you all my information for your state of interest, and you can use that as sort of a starting point.
I also started the same sort of project for the various US Governmental Departments, but again, mostly "harvested images", and never got around to constructing introductory "pseudo-monographs.
I personally also collect these envelopes with corner cards of USPOD Departments, mostly from the 19th century, but have never organized them into a monograph of this nature.
I know one collector who collects these OB business envelopes for all US Government Departments based primarily on the variations in the penalty statements or penalty indicia, then the corner cards. I am told he has thousands or maybe tens of thousands. It is what you might call a "target-rich" environment.
By the time the USPS replaced the USPOD, most envelopes of this nature were prepared at the local level and not part of the ordering process from USPS Supply catalogs, so the variety is literally "infinite". If you decide to collect at this level for any federal department, buy lots of cover boxes. Sadly, most current envelopes, and many early ones, are legal size and not as easy to organize and manage.
I also would be curious to learn of others who might collect some phase of this topic and how they approach it.
MikeL