---

Recently, I ran into more of Pierre D's photos of this specimen (at this link).

And one photo there provides a clear view of the face...and thus removes the ambiguity near the end of the keying process using Curran(1938) as presented in my comment above.

Here's the photo:



...and continuing with the keying sequence given in my comment above...starting from where the choice was previously unclear...we have:

9. Face with one or two vittae........10
10. Face with two brown vittae which meet above and below......gambiana Saunders

So the ID of Monoceromyia gambiana seems unambiguous now.