Wico handles are known to fit in P-360s.
A lot of us think the SlikStik P360 handles ought to fit Wicos, but Christian says they don't. I've asked him before, but he's never said why they don't fit- diamater just a hair too big, e-clip groove in the wrong place, or what. Some of us have wondered if they might really fit, and he's just never really tried it out, or maybe they would fit with some minor modification (like dremeling a new e-clip groove or a couple of washers or something).
I wish somebody with both would test it out.