The 320 fountain pen and the 380 ballpoint belong together and were made between 1971 and 1977. Dating an individual pen to the year is not possible, since pens of those years carry no date. For placing the post-war models, the standard reference is “Collectible Stars” by Jens Rösler and Stefan Wallrafen, which covers production from 1946 to 1979 with illustrations, production periods and estimated values.
The figures given there should be treated with caution. Printed estimates date quickly, because the prices actually achieved fluctuate widely and depend on condition, completeness, nib width and colour. Collectors therefore work mainly from completed sales of comparable pens rather than from price lists; the real use of the literature lies in identifying the model and its production period. The original case and papers do affect the price obtainable, though with the plain plastic cases of the 1970s only modestly.