Italian laws are quite strict about what car you can drive when you pass your test (which I think is based around the engines capacity). Therefore, to give the Italian youngster something with a bit more go (and they can spend more time revving the nuts off) they put the 6 speed box in with the 1108cc engine.
Basically, the ratios in 1st and 2nd are almost the same as the 5 speed box, and 6th is the same ratio as 5th in the 5 speed box. However, 3rd, 4th and 5th are stacked together where 3rd and 4th are in the normal box, giving the driver more chance of being in the power band (with the 55 engine the term power band needs to be interpreted loosely
).
The other benefit being the 6 speed box is not the usual Fiat 'loose spanner in a bag of bolts' affair and has a much, much better feel and more positive change to it.
The 55EL 3dr was the same spec as the SX minus the electric windows and central locking, but plus the body colour bumpers (and a few other 'sporty' bits I think). The 55EL was later replaced by a 55SX 6-speed which was exactly the same specification as a normal 55SX. Never seen a 55SX 6 speed but I have seen a few EL's from time to time