The Blue Book of Airguns lists under the 'German Diana' heading (Mayer & Grammelspacher) a Model 100 which is a .177 target model manufactured 1989-98 and is a single stroke pneumatic. However this cannot be what you are looking for since the period is too late. There is no mention of a Milbro Diana from the time (30 years ago?) when you acquired this info. although I am sure other members will come up with something more meaningful.