Ah, I remember having one of those...still have the burn scar on my left bum from sleeping without extinguishing the fire. Good thing the mattress didn't catch fire or anything.
Good ol' memories. (I used that kind of warmer because warm water bottle would get chilled and wake me up in the middle of the night. This was before our family finally bought electric blanket, since the neighboring house down the block burned down from leaving one on during their outing.)