1969 Proof Sets.
I've always liked the S-mint proof sets quite a bit since they (mostly) have different mint marks than circulating issues and are very high quality. I don't like them as "investments" however because mintages were sky high and attrition very low.
Things have been changing in the last couple decades because most of the coins have tarnished in the older sets and few people are doing anything about it. Most of these (nearly 90%) can be cleaned with a soak in acetone but the longer they are ignored the lower this percentage drops. Only about 50 to 60% of the older sets have been lost or destroyed over the years so they are still very common but now the coins are tarnished. Lest anyone think the coins from the dismantled sets are still around I'd remind them that countless millions of S-mint proofs have been recovered from circulation. Many sets have also been lost in flood, fire, and misadventure.
The '69 is my favorite for several reasons but chiefly because nice specimens of many of the circulating versions of these are very tough in any grade now days.
I've put little effort into proof sets but over the years have cherry picked about 500 sets. I just cut up these 100 sets to clean them and check for high grades and varieties. All of the quarters, dimes, and cents needed to soak. 80% of the nickels and 90% of the halfs also needed attention. There were four nice frosted halfs, two quarters, three dimes, one nickel, and two pennies. There was one nice variety; a better DDR nickel. 90% of the coins were salvageable. 80% of the salvaged coins are nice attractive specimens.
If all these numbers were typical it implies that about 35% of mintage are still attractive coins. Since they were cherry picked I'd guess the true number is closer to 30% or a little lower. I believe this number would be ample even with a mass market but if the market were to develop very quickly then getting marketable coins available would stress the wholesalers. Demand on these is much higher than for mint sets but still puny compared to the availability of (mostly tarnished) sets. There's already a market for chPR rolls but this market is very shallow and at low prices.
I suppose the biggest shock here is that out of all these coins only three or four are candidates for grading!