A bit of reverse fettling when I got home this afternoon - I took apart a solar powered shed light that stopped working some time ago to see whether it took a rechargable battery such as an AA or AAA that could be replaced. This is what I found, never seen one of these before.
View attachment 725513
Could be 3 AA or AAAs wrapped up in that which are generally c. 1.2v each as a rechargeable.