The kitchen area outlets, all 2-prong, seem to be original to our 1950s California house, or perhaps modified in the late 1960s. In preparation for installing a grounding wire "bus" to properly ground the outlets, and convert them to 3-prong GFCI outlets, I (a) took off the faceplate from one counter outlet to see what I could see (photos below) and (b) replaced the faceplate, adding a 3-prong to 2-prong adapter with the grounding tab secured by the faceplate center screw. I then took readings with my new Ideal circuit analyzer (photos below).
Those analyzer readings to me suggest that the metal junction box holding the outlet is in fact grounded to the electrical panel somehow. The wires running to the electrical box are 1950s/60s non-metallic cable of the era, without metal armor or conduit. I believe they do not contain a ground conductor, but am not sure. However, a bare copper grounding wire may also run to the kitchen area (it's hard to trace).
Do my analyzer readings (particularly impedance) indicate my outlet junction box is indeed grounded to the panel?