They made 30s at some time because the disconnect for my water heater had them in it when I bought this house but they were flaky. I ended up chucking the whole mess and buying a regular SqD breaker style disco when I moved the water heater.
Over 30 ago I sent Harold E some of those plug fuse breakers because he couldn't find any in New Jersey.
My experience here with S adapters was fairly negative. They had installed the size that would take a 20 or a 30 and the whole box was stuffed with 30s. Needless to say most of the conductors were 14 gauge. They were blowing the 30s. I finally talked the lady into simply getting rid of that fuse box and putting in a new panel. (a neighbor)
They ran a few extra circuits for the window shakers and the overloads stopped.