Pressure switch for Ultrafire 502B - is there one?

I can't find any...

I use this one:

http://www.manafont.com/product_info.php/ultrafire-pressure-switch-tail-part-replacement-55cm-p-4999

And mount it myself in the original tailcap.

Good idea.

What are the three bulges in the handle for? They all respond similarly or is there some kind of mode selection using each?

Nah, just 3 separate contacts so it's probably more reliable. All three "bulges" do the same job... to complete the circuit. Nothing fancy, good quality and awesome price compared to elsewhere. Bought 8 of them lately. Very satisfied. sucessfully mounted in all 18650 sized flashlights so far that had the metallic ring that works as a switch retainer. Not the best choice of words but bare with me as i'm not a native english speaker. I'm quite sure i seldom write some heavy nonsense. :)

A friend of mine who is a native Afrikaans speaker works on a national newspaper here. He is appalled by how badly the native English speakers write in their own language.

There's nothing wrong with your English Budgeteer, it is a lot better than a lot of what I see every day from people who can only speak English and some Scots.