This part was introduced in 1915. Like many parts, there are several variations, especially in the early parts. The most significant change was from 56 teeth originally to 57 teeth in 1921. Several of the early 56 tooth versions had steel faces, and these were nickel plated. An example of a nickel plated steel 27a is shown at the bottom of the illustration.
Once again, here is a puzzle. The top two gears are opposite faces of 56 tooth gears, with the same pitch and same diameter as the obviously genuine Meccano 56 tooth gear. These were found in US Meccano outfits, but this does not mean that they are really Meccano parts. The two top gears have bosses made from what appears to be a zinc alloy, swaged to hold in a square hole in the (quite thin - 0.033" or 0.84 mm) gear face. The gear face appears to be stamped, while the "real" gear face is machined and 0.062" (1.57 mm) thick. The bosses are all tapped 5/32" BSW - the standard Meccano thread. I think that Don Redmond said it quite well: "If it looks like Meccano, and screws like Meccano - - but the analogy with ducks is pretty feeble".
29 July 2001 - Two of these odd gears have appeared in a ca. 1920 US No. 5x Outfit; the No. 5 outfit is supposed to have two part no. 27a, and the Inventor's Outfit has one (the 5x was a No. 5 plus an Inventor's Outfit). The outfit is fairly "clean", that is, there are few other parts added. This lends support to the possibility that these are indeed Meccano parts.