This part was removed from a 1965 Starcraft Suncruiser 20' w Mercruiser 150 HP GM 4.1L 250ci Inline 6 Dual Carb System w Kiekhaefer Mercruiser 1. 

This is a: 13-14 Gallon Metal Gas Fuel Tank - I am not sure of the exact gallons but here are the approx dimensions: 18 3/8" wide x 16 3/8" deep x 12" tall.  This has some rust in the tank and will need some flushing before final use.    

Stock No: 00G017-JB - 18.6