I finally found some LED replacement lamps that actually seem to work right with the reflector in old-timey flashlights (including the right-angle military MX-991/U) that were designed for the traditional incandescent bulbs...

A nice upgrade without any dead spot in the middle or other interference pattern in the beam.

These are for two-cell flashlights.  I think they also have some similar ones for bigger flashlights with more cells.  I think the key is the "glass" has the diffused collar to put light into the reflector smoothly.