Initially when I hocked the circuit up on my breadboard the crystal seemed
to oscillate at around 27 MHZ.
Then I built the little circuit on a permanent board and now the crystal
oscillates at about 9 MHZ and at this point I start to ask myself what am I
measuring here. And what assures me that 9 MHZ is the fundamental?

Uwe-

As has been pointed out, you probably have a crystal from an old CB set,
designed to operate on its third overtone. At one time CB sets were sold with
only a few channels. You purchased transmit and receive crystals for the
channels of your choice, to supplement the one that was supplied with the
radio.

How do you know the 9 MHz is NOT an overtone? The observation that crystals
usually only operate at an odd overtone, might lead you to believe this. For a
9 MHz overtone, the fundamental would likely be at 9/3, 9/5 or 9/7. There may
be odd multiples of these frequencies that fall near the observed 27 MHz
frequency, but they would be relatively high multiples.

One way to tell would be to incorporate tuning components to select a desired
frequency. If the crystal will oscillate with tuning set to 9, 27 and 45 MHz
but not at 3, 1.8 or 1.286 MHz, then it is reasonable to assume 9 is the
fundamental.

73, Fred, K4DII