I'm not sure of the actual location of the fringe, but from St Erth 'box towards Plymouth, it's Track Circuit Block (using axle counters) controlled by Roskear. Towards Penzance, it's Absolute Block to Ponsandane and from there, where the line becomes single, it's TCB▸ to the terminus. I think I remember correctly (from more than 50 years ago) that the Down Main line to Ponsandane is track circuited, so that the Penzance signaller can give "train out of section" to St Erth as soon as a down train clears onto the single iine towards Penzance, without having to wait to see the tail lamp as the train passes the actual signal box at Penzance
AFAIK▸ it's still AB from St Erth to the first stop signal controlled by Roskear Junction with a tail lamp camera provided to allow Train out of section to be sent once the train has cleared the section.
Unfortunately the Down Main from St Erth to Marazion did not become fully track circuited in the 1974 scheme, the 1974 Sectional Appendix says that for any train heading for Long Rock depot or No 1 siding, the guard must inform the Penzance signalman whether or not the train is complete with tail lamp. I'm not sure why a tail lamp camera was never provided here since when St Erth was switched out late at night it could be quite a restrictive arrangement.