I have a Lucas D57 8" hreadlamp which I bought from a retired MMM man. It is not dissimilar to the L140 and was possibly an earlier model. Were these ever used on MMM MGs?
Are you sure the model number on your lamp is D57 and not R57? If it is a R57 it fits late 1920's Austin Heavy 12/4. I've never come across a D57 lamp.