Diana Model-48E - Late Model.
I think this is a later model rifle with a straight cocking lever. Many thanks to Stan for permission to use these pics.
Other features of note include that the 'made in Germany' on top of the cylinder appears to have been filed off, although the same statement hidden under the barrel is intact. Also, the Diana goddess symbol is of the earlier variety, with leg showing, and it has 'Diana Luft-Gewehr' on the barrel, which was phased out sometime after 1931, suggesting this rifle probably dates from the early '30s at the latest.