I was also interested in knowing which type of aircraft but can’t find this detail anywhere, none of the booking sites give this information, if you were to go by last years ZT331/ZT332 schedule it will be a mixture of B757, B737 and A321 but I could of course be totally wrong.
The aircraft schedule is:
ZT331 departs Stansted at 08:10 arriving Chambéry at 11:00
ZT884 departs Chambéry at 12:10 arriving Luton at 13:00
ZT883 departs Luton at 14:40 arriving Chambéry at 17:30
ZT332 departs Chambéry ar 18:30 and arrives Stansted at 19:20
Hopefully someone else will have better luck !