I bought a NiteIze 2-4 C/D cell 'drop-in' for $6 at Target. I put it in mt Mil Surp MX-993/U, a right angle headed 2D cell OD flashlight made by 'Fulton'. I bought this thing years ago since it reminded me of the similar MX-993/U grey flashlights, also made by Fulton, which we had on the sub tender I was on '69-'72 - the USS Fulton (AS-11) - yeah, hokey reason. BrightStar as a maker is also common.
Anyway - turning the flashlight on with the drop-in in place seems to make the room darker. Seriously wimpy flashlight. I checked the D cells: 1.46-1.47V. Brighter with new cells (1.61V), large & relatively flat (especially compared to the original incandescent bulb's small brighter spot) - with a minor central spot - but still, overall dim. In fact, a 2D MagLite LED drop-in, which wouldn't fit appropriately to make contact in the MX-993/U, when in my one 2D MagLite, will produce a slightly larger and brighter central spot than the incandescent cell, with a larger, and about as bright, outer beam than the Nite-Ize drop-in. While nothing like a 3D cell LED MagLite, the 2D cell MagLite drop-in will significantly improve an old 2 D-cell MagLite.
Stainz