Old Darby Leg Irons

Hi Folks,  I'm looking for some help identifying a set of old leg irons I aquired about 15 years ago and which I'm planning to sell.  They came closed without a key but I was able to make one (though not in the original clock key style) that matches the 14 thread per inch bolt.  Originally plated, some of the coating survives and does so in best condition where the bow enters the barrel - this is perhaps because they were locked closed for many years.  To my eye the plating looks more like nickel than chrome but I could be wrong.   Although there is some corrosion damage careful examination reveals no identifying marks anywhere on either cuff.  The chain is hand forged.  The springs are strong and the mechanism of both cuffs is in good order, they were certainly built to last.  Supposedly they came from Durham prison, but I've no proof of this.  Any ideas on their maker would be welcome.

Hi Andrew, the only unique things I can see on your leg irons are the smooth (non stamped) end caps and the swivel. If someone has a matching manufactured pair that is stamped with a makers mark, that might be a way to ID your irons. I will check my records for which manufacturers made 14 TPI keys.