OK: the 400 uses the 750 filter housing (-300-) and the 350 uses its own (-333-) ... or apparently one from a CB650Z (?) fits as well (-426-).
The "bolt" (bypass valve assembly) is also different, again the 400 uses the CB750 part and the 350 has its own -333- one.
I don't know what the difference is and I believe I've freely interchanged them, but maybe that's a false memory.
On 350 and 400 bikes side by side I have never seen a difference.