MGA was available with a variety of headlamps, depending on regulations of the destination country. Since the large majority of MGA were exported to North America, I will start here with a little U.S. History of automotive lighting.

In 1939 USA required all new production cars to use sealed beam lamps for headlights. In 1940 a consortium of states settled on a single standard for 7-inch round headlamp bulbs to be used on all cars. In 1957 this was amended to allow quad lamps (two each side) in 5-3/4 inch diameter, one having high beam only and the other having high and low beams. In the sweeping safety and emissions standards of 1968, the sealed beam lamps were written into federal law. In the early 1970's this was amended to allow two sizes of rectangular sealed beam lamps comparable to the prior round units. In 1978 Halogen headlamps (in the same configurations) were allowed when halogen sealed beam bulbs became available. In 1983 the law was changed to allow a plug-in bulb in a separate lamp housing, and the housing could be almost any practical shape for body styling and aerodynamics, and it could be plastic for the first time.

There we have full circle from plug-in bulbs through sealed beam bulbs and back to plug-in bulbs. MGB happened to be in production during the (allowed) change to rectangular sealed beam bulbs, but there was never any body styling change of MGB to actually use the rectangular bulbs. So the MGB (and any other MG being sold in North America) continued to use the standard round sealed beam headlamps through end of production. As such, All MG cars in North America from the MG TD through the MGB and MGC used sealed beam headlamps as a matter of law (excluding MG TC that was never formally exported from England to North America). That means any headlamp that is not sealed beam is technically illegal to use in MG cars in North America, including our beloved Lucas PL700 Tripod lamps. Not to worry too much on that last point though, as most police officers as happy if you just have headlamps that actually light up.

For the rest of the world (and the minority of MGA) the story is much different, with plug-in bulbs being more the norm. The following parts are found in the MGA Service Parts Lists.

        MGA 1500
  7H5483 Light unit, left dip RHD
17H5375 Light unit, right dip LHD
  7H5485 Light unit, vertical dip LHD or RHD
47H5124 " " " " " New part number Com.(c)58918

        MGA 1600
  7H5483   Light unit, UK RHD
47H5528   Light unit, Sweden, Com.(c)72040
47H5124   Light unit, LHD Europe except France
47H5124   Light unit, LHD France
17H5375   Light unit, right dip, LHD except Europe, Fin.(c)105361
17H5375   Light unit, right dip, LHD except Europe, Com.(c)105362

57H5010  Jun 59 Headlamp, USA - (Mark VIII S/B)
    Com.(c)70222 Fin.(c)102856
57H5382  Jul 61 Headlamp, North America - (Mark X S/B)
   Com.(c)102857 (USA), Com.(c)107031 (Canada)

So aside from North America we have vertical dip, left dip, right dip, clear lens or selective yellow lens (France), and a difference of connector plug type, most (if not all) of which were separate plug-in bulb type headlamps. Check the SPL carefully to determine which headlamp is appropriate for your MGA depending on current country of residence.