This item is mostly a curiosity, but could be a concours concern. The MGA heater had two different designs for the outlet door assembly. The first one has air direction baffles attached to the moving door. The baffles have small fingers on the end that catch inside the housing to stop motion of the door at about 75 degrees down when open. The later model has air direction baffles fixed to the housing, and the door will move to about 90 degrees down when open.

The illustrations come from the Service Parts Lists. 1500 and 1600 SPL's illustrate the early model, while the Twin Cam SPL illustrates the later model. 1500 and 1600 SPL's show the same part number for the outlet door assembly, while the Twin Cam SPL has no part number given. From field reviews so far, I have pictures and reports of the early style parts in MGA 1500 at least up to 1958 (or at least late 1957 production titled as 1958), and pictures and reports of the later style in some 1600 cars. The part number of the assembly did not change, so the later model supersedes the early one for service parts. Since the later style is illustrated in the Twin Cam SPL, this infers that the change happened approximately with or before introduction of the Twin Cam car in April 1958. Therefore some late production 1500 car and all later models should have the later style heater outlet door. Now it is a matter of narrowing down the date of the production change. Look for cars in the range of (c)37000-(c)52000.