How interchangeable are standard parts?

[Mr. Whippy]

For example, will a AAA Fenix head fit on a Maratec AAA body?  I'm thinking more along the lines of how swapable are tailcaps...

[space cowboy]

I read that ITP & Meratec parts are interchangeable. I don't really know about other brands though.

[NutSAK]

I read that ITP & Meratec parts are interchangeable. I don't really know about other brands though.

iTP and Maratac are both manufactured by iTP, and are slightly different version of the same light (same circuitry and emitter).  They are indeed interchangeable.

The only other swappable parts that I've commonly heard of is the Fenix L0D (or LD01) and the Streamlight Stylus, I believe.  Some folks use the Fenix head on the Streamlight's body.

I've not heard much about swappable tailcaps.  Usually the diameters, thread style and pitch don't match up from one manufacturer to another.