The fine print says "Capacity 12 gallons plus oil", I believe Tempo (and others) made tanks for permanent installation in various shapes. I have seen capacities up to 18 gallons. Triangle Boats and Motors on Gratiot Ave. in Detroit was a Tempo distributor as well as an Evinrude Parts distributor for Michigan and northern Ohio and Indiana. Tempo also offered a piston pump that would screw on to the fill fitting which could be used to refill smaller tanks. Many of the various fittings used may now be found in the Sierra Marine catalog.
Those of you that received a reprint of our first Outboarder magazine will note the back page contains as ad from Triangle Boats & Motors listing there stock of trade-in Elto Ruddertwins for sale. When I was assistant Parts Manager their in the mid 1950s, they had a couple dozen stacked in the basement. The founders, Jack Furgeson & Phil "Frenchie" Cory, retired to Florida in the late ’60s. They used the money from there 40+ year Outboard business to build a big apartment house in Miami that kept them secure in there retirement. . . 😎