I had some ferrite cores in a envelope marked FT50-43. To check that these
cores are actually FT50-43, I put a test winding on one and calculated the
inductance. Using a Booton 260A Q-Meter, I tried to verify this. I got no
indication over a wide frequency range on the Q-Meter. The Q-Meter stayed at
zero.

Doing the same thing with a FT50-61 core, the Q-Meter gave a accurate
indication. Is there something about the 43 material that does not allow a
indication on the Q-Meter?