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I'd call your tank a toaster style. To my eye, they are a little more on the rectangular side and maybe more like a slightly leaning rectangle ... a parallelogram when viewed from the side.  I consider the 1980 Suzuki GS 450ST as having a great example of toaster style gas tank. They are slab sided and usually hold more fuel than all other tanks. Probably the most famous "toaster gas tank" was on the BMW R75-5. It was even chrome I believe. There is a hilarious story connected to the red toaster tank I posted above and I'll try to come back later with that link.

No eBay APP ID and/or Cert ID defined in Kunena configurationYou can get a ugly mofo like this one. It's bigger. 
4.8 gallons
Steve