Yes, it was. You can now get H4 bulbs that go in the standard incandenscent base (t45 base?). I have this, with the curved lens, and they work better than the normal bulb. I've not looked at a flat lens assembly, but the internal reflectors must be different as well because in this case the W113 had an incondescent low beam and a halogen high beam. An interesting fact was brought up that the sedans could be had, at this time period, halogen for both low and high beam. For some reason, that was not an original factory option on the W113, only on sedans.
Rodd
Powell, Ohio, USA
1966 230SL, Euro, Auto, Leather, both tops
1994 E420