@delta558 I stand corrected. Thank you for the clarification. That explains it nicely.
I've since found the Piper Turbo Arrow IV Information Manual, which explains this further:
https://static1.squarespace.com/static/58fe2dcfff7c500a04eda841/t/5c330a8b032be4fbaf385d84/1546848957043/AFM_HBPPB-compressed.pdf
For information, section 7.5 in the above manual provides the following details:
The turbocharger control system is a fixed, ground adjustable orifice ("fixed wastegate"), and is adjusted to provide 41 inches MAP at full throttle at 12,000 feet density altitude. Throttle position controls engine power and no separate turbocharger control system is utilized. An overboost valve prevents manifold pressure from exceeding 42 inches Hg. should the throttle inadvertently be opened too far below 12,000 feet density altitude. Should this occur, the amber "overboost" warning light in the annunciator panel will illuminate.