The ½" pulley with boss, part 23a, was introduced in 1915, according to The Development of the Meccano System (known as DMS - for more information on my Meccano References, click here). Over the early years, there were several variants, in part at least due to the shortages of brass during World War I.
Above are the examples I have found of this part:
From left to right (all single tapped except the last):
||Turned brass, 0.55" outside diameter (almost 9/16")
||Brass boss 0.374" diameter, 0.386" long over peening; steel discs about 0.023" thick; nickel plated
||Brass boss 0.375" diameter, 0.413" long; steel discs about 0.047" thick; nickel plated
||Stamped steel boss, discs 0.020"-0.023" thick
||Turned brass, 0.625" outside diameter (5/8")
||1153a or 1153b
|| As number 5, but double tapped
One of the difficulties in examining the history of part 23a is that it was not included in the regular series of outfits until the No. 7 of 1922 - but the No. 7 outfit was not sold in the US, and was not introduced in England until 1922. The Inventor's Outfit (1915-1920; Inventor's A 1921-1922) includes one 23a, and that was the only outfit sold in the US containing a 23a. The Inventor's Outfits are not common, so the ½ pulley with boss is not easy to come by.
In DMS, the earliest version listed has "thin steel discs gripped by boss", and is listed as Ref. No. 1151. In the 1914-15 Meccano Prize Models manual, the part is described as a "1/2 in. pulley with set screw (turned)." I therefore suspect that the leftmost pulley shown above might be a real Meccano part, even though it is not listed in EMP.
I do not believe that any of these parts are unique to US Meccano. These are simply the versions I have found...