Do you have a Vanderveen`s book? There is a chapter about measuring primary inductance (indirectly, by meas. Lsec) in OPTs. Plitron specifies Lp of their OPTs on 230V/50Hz, I doubt that there is a standard? Lp is not constant, but have "parabolic-like" curve in respect with rising voltage. Maybe measuring a few points to find L/V curve and then specifing L on some "real" voltage, useful in grid choke function, say 50Vrms?