That is not a Sweet Sixteen serial number.
Your Browning sweet sixteen shotgun was made by FN of belguim for Browning in the year 1951.
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The Sweet sixteen name is the model of your Browning auto-5 shotgun!Is the number 51666 the serial number of your shotgun?Please re-post your question so that a answer can be provided.
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Your browning sweet sixteen shotgun was made by FN of belguim for browning in 1961.
That's not a Sweet Sixteen serial number. In fact, it doesn't even look like a Browning number to me.
Your browning 16ga auto-5 shotgun was made in 1934 with the serial number that you provided.However the browning sweet-sixteen auto-5 shotgun was not offered until 1937.These sweet-sixteen shotguns were made from 1937-1975.Does your browning shotgun say sweet-sixteen on the shotgun?If so please check your serial number again.
Value of Browning Sweet Sixteen serial # 74366