My 15t came from spares for the Sachs/SRAM 3x2, a 13/15 pair 3/32 wide.
they made the short cassette using the same 3 tab spline..
newer Sturmey BWR adopted the Shimano BMX hub spline,
haven't laid hands on them , spline for the derailleur cassettes has 1 wider than the others.
to align the cogs shifting details to each other ..
perhap all splines are the same when a single cog Bmx setup..