Are you talking about the MX-991/U with the colored filters, spare bulb, etc? It literally is an inexpensive flashlight with some O-rings. I tried a Nite-Ize 2-4 C/D drop-in - it is very voltage sensitive, reportedly working better on 3-4 cells. It really has a flat, and dim, beam, with a miniscule spot. The 2 cell LED MagLite drop-in won't work without modification due to the negative contact. That light is best focused, a la the MagLite, and produces a larger and seemingly brighter spot than the original incandescent bulb - with a wide diffused light similar to the Nite-Ize. I will keep mine as it is, with the wimpy Nite Ize - and the red filter at the ready - for star parties (Astronomical loonie gatherings... at 'dark sky', as if they still existed, areas - with telescopes.).
At least the Nite-Ize is cheap - ~$6 closeout at Target - ~$8.48 @ Wally World.
Stainz