3R12 is a 4.5 Volt battery still sold in Europe from Duracell, Energizer and Varta. Duracell even brands their own flashlights that uses 3R12 batteries.
"Wonder" is the French manufacturer that, starting in the '70, flooded Europe with those flat flashlights and batteries. I still have some, metallic and plastic.
On a note, those 4.5 Volts alkaline batteries are not manufactured to the state of the art, and are relatively expensive too. Their capacity in mAh is lower than 3AA alkaline batteries in series; so I am using the empty plastic case of a 3R12 battery with a 3AA battery holder inside, with much better results.
You can also stick a protected Li-Ion in a 3R12 case and make a very good rechargeable battery.
Hope this helps
Anthony