On 1 November 2024 at 10:42 Nigel Graham 2 Said:

…
I also had something really weird. A twist-drill of perhaps 4mm diameter, whose helix reversed from right to left hand half-way along its length. It looked as-manufactured, not someone’s idea of a joke by modifying a standard drill, and I though I know there are LH helix drills for special purposes, I haven’t the foggiest why a hermaphrodite one would be made!

Just a guess, but specials made to speed up mass-production weren’t unusual.   They allow two jobs to be done with one operation, avoiding a time-wasting tool-change.

Maybe this example drilled through a body into a cavity and then carried on drilling on the other side.  The reversed helix would keep swarf produced by drilling the second hole from climbing the drill into the first hole and jambing it.  Swarf from the first drilling is ejected normally at the top, but swarf from the second drilling is kept in the cavity.

Dave