Very interesting car. Based on the pictures your car appears to be a wide-bodied Plus 4 roadster, which began with the 1958 model year (October 1957). But most 1958 roadsters have the straightback rear with the low gas tank. (The bustleback rear was necessary to accommodate the high gas tank which was discontinued in 1958, except in drophead coupes.)
I have seen at least one other wide-bodied bustleback +4, and I think these are very early 1958s, made in late 1957, when Morgan transitioned from narrow-bodied bustlebacks to the wide-bodied straightbacks. Morgan probably made the transition in 2 steps: First, the bodies were widened, along with the wider bonnets and windshields. After successfully producing a few dozen wide-bodied bustlebacks, the factory then modified the rear to the the straightback form.
I think it would be correct to call your car either 1957 or 1958. Prior to 1970, in the USA it was common for registrations to be delayed for a year or more.
