With thanks to Doug for these pics of this rare rifle. It's in .177 cal. He reports that the stock was damaged and repaired at some point before he took possession of it and that there is no buttplate. The rifle lacks a rearsight too.
As you will see, there's a strange '106' stamp to the right of the DRP marking on the rear left of the cylinder. According to Doug this doesn't correspond with the possible serial number stamped around the loading tap, which he says is "a three digit number under 300".