I have an Atkin spring opener. I spent some time in finding it (I particularly wanted an Atkin). Mine was made in 1911.
A point to note - I handled a number - and they had rather different 'feels' on closure - varying from smooth and easy to really quite hard to close. I did some research.
The problem would appear to be that this action, like the Beesley Purdey (which is near identical) needs to be 'just right' if the closure (which tensions the mainspring which does both the tumbler and self opening) is to be smooth and uniform. If it is not right, the force is not uniform through the travel of closure and the closure action becomes stiff/hard/lumpy. It is (I have been told) very hard to get right once this has been upset. A good one is a joy to use (as Gough Thomas wrote), but there are ones about in states that are not anywhere near what they should be (Gough Thomas described them as 'lumpy' in closure feel).
Try and handle as many as you can (and also Purdeys) so that you become familiar with the 'feel' of that type of self opening action.
I have handled some that I would not have wanted to own/use.
