Just to have a little fun with this...the 575.0 you initially measured using Audacity is very close to GE 577.5 "G05". The closest GE tones to 600.0 are 592.5 "G06" and 607.5 "G07", making it likely that it is indeed Motorola 600.9 "121". It's entirely possible that they are using a mixture of the tone plans, GE and Motorola QCII. You should see some of the bastardized tone combinations used in Rockland and Orange, it would make your head spin! Mixtures of Motorola QCI and QCII, Plectron, GE, Federal, Reach, Bramco and even a few completely custom tones all mixed together, sometimes in the same two-tone sequence, with no rhyme or reason.
While I agree with Alarmguy that Audacity is usually off by a little bit, it may've been closer than you think in this case. I don't think it would nail the 600.9 second tone almost exact, but be off by almost 6 Hz for the first tone. The tones are too close together for that much variation between them. Of course, a lot of factors come into play when measuring them in Audacity, not the least of which is the quality and format of the recording you're measuring, as well as what settings you're using in Audacity to measure them. The PL/DPL can also throw the tones off a bit.
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