I have always wondered what percentage 'new' material ('new' being defined as material which has not been seen anywhere in the marketplace in the last 50 years) is actually presented annually in our hobby.
Well none of my material would qualify as new, yet I have been known as a block hole for the areas I collect, what I get never is seen in the light of day again. It wouldn't be until 2033 for my Q and JQ material to age to "new" status. My QEs would turn new a few years sooner.
That said I have Q/JQ material which was "new" (sat in two suit cases in an attic from the late 1930's until after 2005) with my half still not seeing daylight except during the non-public private treaty hand off. The other half has been seen a bit in an exhibit and in one public auction years after the non-public private treaty hand off. My buddy and I bought it together and split it 50-50. That was from a collector's accumulation put together in the early and mid 1930s.
Back in the 1980s a series of covers and tags from the 1913-1923 range came out of the California Sierra foothills in no organized fashion to be sold at auction through the Henry Spellman (of San Anselmo, CA) public postal history auctions for years. The covers/tags were all gleaned from the files of the California Ink Company's Berkeley manufacturing facilities and held until dispersed through Spellman. All values of the 17 were present except for the 50 cent and $1 red and 25 cent green.
So in the last 39 years that is the two of which I am aware and Henry Gobie who study Parcel Post for years prior both exhibiting as well as writing the book US Parcel Post: A Postal History knew of no such finds by him, Doolittle nor Lightfoot. The few reported/recorded in the press of improper 1912 usages when the stamps were invalid have disappeared. Except for one example which resurfaced from being buried in the suitcase holdings.
The Corry Hide and Fur Company along with other names which included Berliner had mailing tags and some envelopes from circa 1900-1940ish hitting the market in large groups. These thousands of items came out of Canada within the last 5-10 years in multiple groups. Most were placed in stock sheets and binders in no discernible order. The holding had very few Q/JQ examples but stamps of many series available during the time range. The ratio was such that I believe the Q/JQ tags were lost to soaking for collecting.
Now new material is coming out and has been for years as the holding of correspondence from WWII has been leaving families and moving into the public collector realm. Korea as well. Still early for the Viet Nam period.
That said, it all is a small percentage on new material entering the market place each year.