PGR 4 derailleur

PGR derailleurs use a distinctive operating mechanism:

The nipple of the cable is held by a fixed ferrule that is at the inside end of the tube along which the pulley cage slides. The cable is clamped at the lever.
The end of the cable outer pushes the pulley cage along the tube.
Unlike pull-chain mechanisms, this allows the derailleur to be top-normal.
A single, tightly wound, helical, spring both tensions the chain and powers the sliding tube mechanism.
Helical springs are often used in compression, but this one is used in tension. The mechanism uses washers with wings on them to engage with the first and last coils of the spring.

Other than this PGR derailleurs are decently constructed alternatives to the, say, Simplex derailleurs of the time.

This is a very, very, well-used example of a PGR 4. Just look at the wear on those steel pulleys! It has the following features:

The main arm is steel.
It has two pulley wheels, which have one thick side and one thin side - very like a contemporary Simplex.
The detailing of the pulley cage plates has also adopted some aspects of a contemporary Simplex. However, the chain is still fed through the pulley cage in the conventional way.
The sliding tube is 50mm long. It is labelled as a 5-speed system.
The main arm is labelled 'PATENT P.G.R. 4 BOLOGNA-ITALY'. I believe that the word 'patent' in Italian does not always relate to a legal IP protection - but often means something like 'model'.