From the examples shown in Hiller's Air rifles book it appears to be a version of the Milbro Diana Model 77. Hiller's pictured example has an identical stock shape, trigger guard, barrel housing and the unusual wrap-around foresight. The only thing lacking is the recoil pad, but Hiller says that "later models appeared to be fitted with a recoil pad". Does your gun have a safety catch on the right hand side of the cylinder, which it would have if it is the Mod 77?