Update: the old Mirage shell is fully plastic. It seems the reflector and the front cover are Sigma's own design rather than a ready-made MR11 package, and the light head has no plug in the rear like the MR11 has. The bulb itself is a 6V, 5W, removable through pulling it straight out through the rear of the reflector. It looks fairly similar to the 6V, 3W (or 2,4W) bulb used in dynamo-powered lights (Lumotec, Basta).
--J