I would consider a reference/master (timebase) oscillator frequency other than a commonly used 10 MHz or a reference which 10 MHz cannot be divided or multiplied by an integer (for the following examples with a 10 MHz input: 50 MHz with a NB3N502 PLL multiplier (set to x5) for the FeelTech FY6600 arbitrary function generator, or 5 MHz with a divide-by-2 counter for the Philips PM5390 signal generator; both pieces of equipment have been modified for an external reference frequency input) to be unusual in test equipment.
Examples of an unusual reference (as per my definition) I have seen (so far) in test equipment include the following:
* Philips PM5193 function generator (8.589934592 MHz)
* Atten F2700-C frequency counter (13 MHz)
* Philips PM541x/PM551x TV pattern generator (5.034964 MHz - applicable only for models supporting NTSC and/or PAL-M (both 525 lines); 625 line PAL and SECAM uses a 5 MHz master oscillator)
* Kenwood SG-5110/5115 signal generator (16 MHz)
If you know of a piece of test equipment (nothing else!) which uses an unusual reference frequency as per my definition, please post it here.