They look homemade to me.
Maybe somebody made them to rough bore cylinders or deep holes.
I'll often use a fly cutter to bore larger holes when the dimension isn't critical. Just bump the tool bit out a little, and plunge away.
Obviously you need a big enough pilot hole before plunging a fly cutter.
Or they could have been made to finish cut the bottom of a deep channel, rather than making multiple passes with a long skinny (expensive) endmill.
I'd be careful with putting too much side pressure on these. Seems like that's a whole lot of leverage to be putting on that small diameter, short shank.