"Sample" Overprint On Scott 191

What does "sample" mean in this context? Why would this stamp be worth less (usually) than a comparable specimen without the overprint?

Thank you in advance

The "Samples" were made by the Banknote companies for the US Post Office to inspect and decide if they met specifications. Somewhat similar to Proofs and Essays but further along the printing continuum. Fewer people collect these (same with Proofs and Essays) therefore less demand and less costly.

Listed in the Specimen section of the Scott Specialized.

Not quite correct. For the contract of 1889-1890 the Government was contemplating reducing the size of the postage stamps. They had two different color schemes for the denominations in mind depending upon the sizes of the stamps. The SAMPLE and SAMPLE A overprints were created to serve as color samples for the two color schemes being considered for those two sizes of stamps. You can read about it in the American Philatelic Congress Book for 1990, "Color Sampling the Contract of 1890: Preparations for a New Series of U.S. Definitives", 55-78.

They are not essays or proofs, per se, but do fall into the larger category of that kind of archival material. While the colors do correspond to the colors of earlier stamps in many cases, these were printed some time after those colors had ceased to be current and were no longer in production for regular postage stamps. Your 90c on soft paper officially resembles a 191, and is listed as a 191SK, but it was produced in 1889.

Speaking as a VERY casual philatelist, I would suppose that the population of stamps marked "SAMPLE" (hundreds? thousands?) would be far less than the general issue (millions)... would this not make the "SAMPLE" overprint variety more valuable? Obviously, there's other variables at work here.

Supply might be low, but demand is low too.

Forgive me for my oversight. You asked about relative value. Some stamps are interesting despite their value, and others are interesting because of their value. I only spoke to the story behind the stamps, and neglected the other part of the original question. Of course, eyeonwall addressed the how of the lower market value of the Sample overprints, but not the why of it. The fact that one stamp appears to be like another does not make it so. A used stamp is not unused. A torn stamp is not whole. An altered stamp is not original. Archival stamps are not regularly issued. Most collectors are quite particular about the type and grade of the stamps they wish to collect. Most of the time a close substitute is just not good enough for what they really want. In that environment, a pristine archival stamp may be seen as a "fill-in" for the "real thing," i.e. the bonafide issued stamp.

So what do you pay for a fill-in? Catalog values give a pretty good picture of how that question is generally answered. Despite the lower value, most collectors do not pursue archival stamps. Why not? Because they are very rare. Not valuable; rare. That 90c "SAMPLE" is one stamp of a set of ten. Try to put together a set from what you get by buying collections. And that set is matched by the "SAMPLE A" counterparts, which are equally as rare. And then there are the omitted overprint variants, and the overprints with manuscript supplementation, which make the normal overprints seem common by comparison. Then you learn that they were intended to be used in strips of five, but that some are known in block form or even as plate number and imprint blocks, which are howling rarities. And so it goes.

Rather than enter that quagmire, most collectors steer clear from even the single example that happens by. They want a 191, not a 191SK.

Demand has a bigger role to play than supply. Sometimes the only thing harder to find than a rare stamp, is someone who wants to buy it.