As Mark mentioned, these are 'rev2' kits - latest spec is 'rev4'
Differences as follows ...
1) Compressor bypass spring replaced by a 'mousetrap' type spring mechanism.
Cost is next to nothing.
2) Original (black box) sub-ecu intercepts MAP sensor signal before it reaches standard ecu.
In addition, the later one (silver box) prevents fuel cut at rev limit to help the pre-cat's stay intact.
Note that pre-cat failure has been a problem on non-supercharged cars too !!
Retail cost of the new sub-ecu is around 600euro + vat
In normal use, the original sub-ecu should be ok ...
but if you rag the nuts off your car regularly you might wanna consider the later one.
2500ukp +vat represents a saving of about 1200ukp +vat on the current list price :o
Tony