The concert harp tuning key fits a 0.185″ square pin.  Many lever harps use that size, but there are two common smaller sizes and one larger size in use.  So, my first question would be “Does the concert key not go on at all, or does it spin around once put on?”

The real way to tell is to measure across the square head of the tuning pin with a micrometer or good dial caliper.   The other common sizes are 0.210″ (Rees harps, drum keys)  0.176″ (Dusty Strings) and 0.160″ (Triplett, all zither pins, some harpsichords).

Any information you can add will help nail down the specifics, like “Do the pins screw in and out of the neck when turned?” or are they smooth, tapered pins like on a pedal harp?

Charles Nix

Harp tuning peg heads are manufactured to fit standard clock key sizes and some older harps (e.g. the Clark, Caswell Bard) do not have the more common sizes.  You could measure them with a caliper and then research clock keys, make a handle and so on but that seems like a heck of a lot of trouble. Others may have been a standard size but have become worn over time and use (zither pegs often have that issue)

An alternative – a bit pricey I admit – would be to buy Dusty Strings’ Universal wrench – which is adjustable.  Frankly, it is just a size #0-1/4 tap wrench with a nice handle, but it will do the job for you….and any other harp that you own.

http://manufacturing.dustystrings.com/harps/accessories/tuning-wrenches/

I keep that tap wrench in my goody bag (along with spare strings etc.); they only cost about $5-$10 at any hardware store.

Good luck!

Biagio