Can't help, but I remember the SL-35 as being a super durable, big, long, metal "cop light" back in the 1980s. Back then, it was just about the brightest D-cell flashlight available: What the MAG should have been.
The SL-35 was a rechargable. Same as an SL-20 but used full-size NiCds in order to support a high-current bulb. Pretty thin walls and vulnerable switch though. The SL D-cell lights were liked a Mag knockoff done under with a low budget.
Take the head and switch off. Shoot some PB Blaster down the barrel, enough to soak the tailcap threads. Leave it be for a while. Then get it nice and hot with a big heat gun or torch. Don't melt it, but get it hot. Whack the tailcap against a scrap of hardwood a few times. After all this it should unscrew without too much trouble. A strap wrench comes in handy.
If all else fails, and you can replace or clean up the tailcap, find a way to anchor the light's body and use a pipe wrench on the tail. I've had a few that needed that treatment.