Allen key size for older HP knobs? [Answered]

[TimFox]

Note that many American turns-counting knobs for 10-turn pots use a Bristol spline key (starry), not an Allen hexagonal key.  A suitable key was usually included with the knob in its original box.  Anything else will ruin the setscrew.

[tautech]

The only better recessed drive type screw is Torx but I can't remember seeing Torx in a grub-screw.

As a Canadian I can't let that one go.  You've clearly never used a Robertson. 

Had to Google that.
https://en.wikipedia.org/wiki/List_of_screw_drives#Robertson
Square drive screws are common in the construction and associated industries and I have used them for many years and they don't come anywhere close to Torx IMO. I've rounded many driver tips on these square Robertson drives too but in saying that, mostly with power tools. Robertson sockets are too shallow and one needs to grind the ever so slight round from the end of the driver to ensure full engagement. Then they last somewhat longer and you don't bugger the screw heads.

The best drive type I've come across is 12 spline, like a double Hex of which the first I saw was in the 70's in the head studs of a 2 litre Ford OHC engine.