Ford dlr says they need # off old O2 sensors to replace them?

[DaTank]

Parts guy said he didn't know what kind I had unless I brought in the O2 # off old sensors. He said he couldn't be sure what to replace them with and it could make a big difference.
Is there not a standard sensor for year and make?
I did a search on 02 sensors, but couldn't find this info., just info on why, how and what kind of problems you have with old sensors.
Thanks
Brad

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1991 Mustang GT Conv.
K&N air filter, Cobra manifold, 65mm TB,
Adjustable fuel pressure regulator, 180 thermo., Mac headers,
High flow H-pipe, Force II W/2 chamber Flowmaster,
Hurst Shifter, 3.55 gears, subframes