I know nothing about what a 501 model is. Is it just a typo?!
It's kind of like "Teh funny"...a typo common enough that it means 510, just like how many people type "belive" instead of "believe" or "loose" when they mean "lose".
OP, I've got a couple of big battery mods and a pile of homemade mods and I still have use for my 510s. I especially love my automatic battery 510, which I pull out when I have an overwhelming craving for an analog, or when I'm watching the BBC comedy "The Smoking Room".
TBL36