I've just looked up the lathes.co description, but it's limited only to the smallest in the American manufacturer's range, designated the '0' and slightly large '1'. The number being that embossed on the casting, with the name.
Nevertheless the text suggests all these machines were small, and fitted with lever-action manual feed, via rack and pinion. Such machines, including the lower ones in the English-made Denbigh'H' series, were designed for semi-skilled production tasks like cutting slots and spanner-flats, on small components.
So the lack of a keyway may have been intended to give some protection against an operator – probably on very tedious piece-work – over-loading the cutter on a stub-arbor that has no outer suppport. An arbor with keyway would be more for the controlled conditions of a machine with self-acting feed, set to give the appropriate tooth-load.
Milling-cutters are all given keyways so they can be used on suitable machines, but that does not mean they always need a key.
The skill in using this machine will be in sensitivity with the feed-lever – and especially on a small machine with ma nual rack-feed, always up-cut the work (i.e. feed against the tooth's direction, never with it).