Hi Bobby -- Your aluminum coin catalogs for less than one US dollar. These coins were minted by Germany as the Weimar Republic after the end of the first World War.
The majority of the dates and mint marks of the series are worth $1 or $2 US dollars at most even in well preserved condition. In fully uncirculated condition, they might fetch $5 US dollars from a keen collector.
However some of these coins were minted in much lower numbers; for 50 pfennig coins in aluminum dated 1919 with a mint mark of D, E, G or J, multiply the above values by two.
The king of the series is the 1919F coin - in fully uncirculated condition, these sell retail for upwards of $50 US dollars. Coins will exceptional eye appeal will be worth even more.
Bobby - the valid mint marks for your 1921 date are A, D, E, F, G, and J. I do not see anything in the catalogs that indicate an F/G mint mark is possible. Of course this does not mean you don't have one. Could it be an E mint mark? Use a magnifier. If you think you have an F/G piece, please start an e-mail exchange with CoinQuest and send us a picture via e-mail.