Not quite - the early 1400s had a slightly different motherboard too. The logic boards in the very first 1400s could only take a genuine Apple CPU module up to 133 Mhz - the 166 Mhz modules only work in the 1400/166 and I think the 1400/133 (and possibly later 1400/117's). However, third party CPU upgrades (including all G3 upgrades) will work in any 1400.
I know that most people are going for G3 upgrades these days (I certainly would if I could find one that I could afford), but still...might be handy to know for anyone who's looking to use a 166 Mhz module in a 1400.