It seems to me that there was a tendency for bush wheels to grow gradually larger over time. I first noticed this with wheel discs; the brass pair of 24a from a '1948' set 3 with October 1948 Manual, new in the Early 1950s, are 1 49/128"; 35.2 mm (just 1/128 over the nominal 3/8"); 3/64"; 1.25 mm thick are noticeably smaller than the zinc pair from an 8X, and the zinc pair of 24c from a 5X at 1 15/64"; 36.2 mm dia, (3/32" over the nominal 3/8"), 7/128"; 1.3 mm thick. The 8X has a 1977 manual, undated box, the 5X has a 1974 manual in a (c) 1975 box, both bought in HK in 1981. There are also 2 zinc 24c bought in UK as spares in 12/1979 which are indistinguishable from the 5X pair. These are still zinc when contemporary bush wheels are already matt brass