Hi Antimatter,
Worked for Telewest - Ntl for approx 10 years mate, its the top of a tratec isolator in the photo I'm pretty sure...
the 2 way spiltters aren't as wide as two main screw fittings in the housing and the 3 ways are too wide to fit in. the splitters also dont have the nylon washers between the main body and the screw top connector (f81). was saying that if you try to put an rg59 f on to rg6 cable you run the risk of cutting through the dialectric (white foam part) and getting the brading onto the centre core - in effect causing attenuation as you've said, but you would not be able to correct the levels without first correcting the f connector.
Cheers