? LED Replacement Bulb f/Lantern w/Old Flashlight Bulb

Perhaps it would be easier to buy an LED lantern. Your post title says "flashlights"

X2 for super bright leds.

I converted some mini-mags years ago, they didn't last long. I don't consider it cost effective to convert old flashlights.

Now I just buy LED flashlights or get the free ones at Harbor Freight.

I got a 3-pack of really nice bright ones at Sam's Club for under $20.

My super bright tactical rechargeable light sells new at flea markets for $15 each.

A flashlight built for LED will probably work much better than a retrofit the same as tail lights.

I have two 4-cell lanterns and replaced the bulbs with these.

https://www.superbrightleds.com/cat/b-series/ looks like they have some in the $5 range- good vendor.