Russ Curtis is correct. Dimensionally, the M38 tub is the same as the 3A tub. The M38 also has a glove box in the same general area as the battery box. If your heater is a Harrison, I don't think it will fit in the standard mounting position.
Although I couldn't get your Craigslist link to work, I found the tub you referred to. It would be a steal at $400.00.