Had recent job where they told me to just give them (4) L6-30 for heaters. So I did and at some point client installed the cord and plug heaters. Didn't think anything of it but on final the inspector takes a closer look on the heater and low and behold it has a max ocpd spec on it of 25A . These were bolt on breakers, and I was actually able to find four double pole 25s at one of the supply houses, but being bolt ons, and with no time to order them online from one of these breaker places, it was an ouch my bottom on the price.Basically means nobody stocked them. Didn’t start seeing any on the shelf until the late 90’s. Still have not come across any used on a water heater, just A/C units.