Series LCR experiment

It doesn't work because they are radically different components.

The PUT is a strange beast; it's like a hybrid between a bipolar junction transistor and an avalanche diode. It has non-linear behaviour: once the current flowing through it has gone above a certain value, its resistance collapses and it becomes "more on", until the current falls much futher than the trigger level.

This lets the oscillator work: it charges up the capacitor, until the trigger of the PUT is reached, at which point it discharges through the LED until the current falls below the turn-on point, and the PUT turns off again.

In some ways this is quite a lot like the 555: you could almost just substitute it with the "discharge" and "trigger" pins wired to the capacitor-resistor junction and the "threshold" pin wired to the 15k/27k voltage divider.

(For practical purposes the 555 obsoletes the PUT, which is why everyone is suprised to find the PUT in a book that's in print today)

In your circuit, the MOSFET will just adjust itself into its linear region such that the gate-source voltage is at the turnon point, and stay there. There's no nonlinearity to produce oscillation.

A simple LCR meter will not give accurate measurements if the core is very lossy.

It it's truly an iron core (not iron powder) then the eddy current losses will be significant- like a resistor in parallel with the coil. That translates to a lower impedance which the LCR meter will interpret as a lower inductance since it is not measuring the phase angle of the result. Iron powder is also lossy.

An LCR bridge would sort this out for you (and show you the equivalent parallel resistance as well as the inductance), but the bottom line is that your core is going to be way too lossy to use at RF frequencies and you should use a ferrite core or air core that is suitable for RF.