Robin Day polyprop non-stacking chair ID?  

Hi there. I recently came into possession of some filthy, but lovely Robin Day poly prop chairs for rather a good price. "There's loads of them!" I hear you cry, however bizarrely these ones are non stacking. I'm keeping them as I love them, but I am curious...

I presume that as they're non stacking they were produced in significantly smaller production runs.

Can anyone shed any light on when these things were made, by who (there's no markings apart from the Robin Day badge on the bottom), if they're any rarer etc?

Thank you