I am not a luthier or a banjo player but I dont see why you could not put it back to 6 string if you wanted a curiosity. This appears to be a 6 string banjo which has been converted to the more standard 5 strings.
Vintage 5 or 6 String Open Back banjo Interesting
57 Vintage Bakelite Or Hard Rubber Clarinet Barrel 56.5 Metal Lined, Adj?
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5 or 6mm Tremolo Arm Whammy Bar for Fender ® & Squier Strat * Black or White Tip
Rico Royal Tenor Saxophone Single Reed Strength 1.5 or 2.5 Free Delivery
Vandoren ZZ Alto Saxophone Single Reed Strengths 1.5 or 3.5 Free Delivery