The Coil issues of the Washington-Franklin period Flat Press printings represent one of the greatest problems in all of philately. Few other areas have so many difficult to detect forgeries. The problem is this: The Washington-Franklins were first issued three ways-as perf 12 singles, as imperforates and as Coils, perf 12. The imperfs and the fully perforated stamps to the 5c value are fairly inexpensive. The Coils are rare. Beginning about 1920, unscrupulous stamps dealers began to cut the perfs off of perforated stamps to make forgeries of coils. These were unconvincing as the margins on the imperforated sides are invariably too small. In the latter 1920’s someone had a series of perforating devices made by a tool and die maker with the appropriate sized perfs. These machines were used on imperfs to forge coils. Thousands were made and today faked coils from this period outnumber the genuine by at least ten to one (once we get to the Rotary press period, from the early 1920’s onwards, there are no faked coils as there was no imperf rotary press stock to work from). US government agencies have only interested themselves in this type of forgery sporadically as the forgery does not defraud the government of any revenue.
There is no other country where such a major period of its philately is so replete with forgeries and this has always impacted on the popularity of these stamps. As it is, genuine coils are very rare but they would sell for far more if their collecting was not tainted by the faked ones. They should only be bought with a certificate or guarantee of genuineness. Even so, I think genuine coils are quite underpriced because of this taint, and if you can find properly certified ones they should perform quite well in your portfolio.
Incidentally, the then owner of the fake perf machines showed me all three machines in his possession in the 1980’s when I asked him to see them. He was at the time the known forger of these stamps though to be fair, most of the forgeries of these coils were made nearly 90 years ago. The machines were small metal punches that looked like smaller versions of the hole punching machines that you use to punch holes for papers that you want to put in three ring binders. He had them in three sizes: perf 10, perf 12, and perf 8 1/2. He died a few years ago, and I no longer know where these machines are.