I just used the link on the suppliers list here on the BB and it worked fine for me. They do have them listed for $385.20. Here's the phone # if you can't get the link to work. 832-632-9289
Steve, Alex is correct - a hole was drilled in the casting before chrome plating.
I doubt though, that it would matter if one were added afterward. The re-use of this fixture that was designed for front signal lights was a "less than brilliant" idea.
The lamp base was never adequately sealed nor is the lens completely weather-proof. Invariably, water works its way into the light and corrodes everything.
In its day - this wasn't an expensive item ... but that day has long past !