Both tanks have aluminum primer buttons so that makes them 1956 or earlier. Evinrude had the same six gallon tank from 1953-1956, and there are lots of them around. The four gallon Johnson tank could be a 1953-1955, or a 1956. The main difference between them being that the 1953-1955 has the words “Mile Master” in white, whereas the 1956 has the words “Mile Master” in red with a white backround.
Here’s a website to see exactly what they look like: http://www.nymarine.ca/index.html
All of these tanks need to be rebuilt, and especially if they got stored with fuel mix in them for years or decades. The tank could be perfect, and still not hold pressure because of a leaky o-ring, top gasket, crack in the hose, or leaky top mounting screw seals. I would rather focus on getting the tops off and the tanks clean, so you can really assess the condition. Probably more important to pressure test after the rebuild.
Improvise-Adapt-Overcome
