After WWII Gecado became Dianawerk. My Catalogue list the Diana Model 35 as being made from 1953 to 1987 under several different Variation's. Without more detailed information it is hard to identify the exact year for you. But considering that you asked for it by it's original name I assume it would be one of the earlier variations.
20 pound
1959
Please check the link for the Manufacture Date codes details on the Remington Society of America.
@ late 1975 or early 1976.
1967-1988
1908
@1970
No way to answer without the serial number.
1977
1972
No way to answer withou knowing who made it.
1969