This is an incredibly rare Abraham Thompson square padlock. 

Many old locks, including this one, have a registration marking that can tell us quite a bit about their age. 

On this padlock we have what’s called a diamond registration mark, and each part of this mark gives us a clue about the padlock’s origin. 

First, at the very top, we have a Roman numeral. This numeral tells us the material the padlock is made from.

In our case, the Roman numeral refers to metal.

Next, at the top of the triangle holds the letter M which represents the year in which the design was registered and if we cross-reference it with a table of registration years it gives you the date of 1859.

On the left side, you’ll see a letter which tells us the month and on the right side, we have a number that indicates the exact date.

Finally, at the very bottom, this number represents the batch number.

So, using this information and the markings on the padlock, we determined that it was registered on 20th May 1859.