Ah, included within a related archive - found by searching under 'content' - I see. D'oh!

Thanks for pointing that out at - I at least have previously searched for AREXX as a package in itself, or rexxsyslib.library in the past - obviously without success. Consequently, I still have an original copy of OS3.9 around somewhere despite no longer having hardware on which to run it (or a desire for emulation).