John Henry was a large wholesaler/retailer; there are one, two, and four cent values. The silk and watermarked paper varieties on all three values are common, the pinks less so and the old papers are fairly scarce. M&M's being expensive is only partially accurate. About 1/3 of all private die proprietaries are readily available in a variety of conditions. The rest range from pricey but easily found to VERY expensive and extremely difficult to find in almost any condition. It is also true that some stamps with low catalog values are very tough to find in fine condition, while some popular and expensive stamps are easily found that way. The only way to know them well is to spend many years looking at as many examples as possible at shows, auctions, and on line retail outlets like
ebay. I've spent 40+ years doing this.