I don't think that the Model 45 has been discontinued. Well, it may have been, but it looks like it is being made again.
If you have Adobe Acrobat Reader, check out the .PDF file of the latest Diana Catalog on the Diana website(see link below). The Diana website was recently updated with this new catalog so it's in fact a new catalog and not old, outdated info. Further evidence that this is indeed new info is that the Models 26 and 28 have the same muzzlebrake as the current Model 24. The older Models 26 and 28 did not even have muzzlebrakes.
http://www.diana-airguns.de/DIANA_Catalogue.pdf
John