Jack,
It is not clear exactly what it is you have, but there are probably many options that you can pursue. Generally speaking, when someone refers to a nickel RTD as a Type A Nickel, that refers to the slope (TCR) of the nickel curve as being a .00672 ohm/ohm/deg C from 0-100 deg C. If the base resistance of this nickel element is 500 ohm at 0 deg C, then the resistance at 100 deg F would correctly be 616.93 ohm. There is another nickel curve that is often referred to as Type B Nickel which changes the coefficient above to .00618 ohm/ohm/deg C from 0-100 deg C. This would result in a 608.38 ohm at 100 deg F resistance.
I gather from the information that you provided, that the RTD was accompanied by a 4-20 mA temperature transmitter that would equate 602.65 ohms as a temperature of 100 deg F. It also sounds like the 40 year old RTD is measuring 596 ohms in some setting, presumably when the process is known to be at 100 deg F.
One solution is to replace both the RTD and the transmitter with a fresh calibrated assembly that will output the correct milliamps that represents 100 deg F in your process setup. In that case, it doesn't matter what the RTD element is, only that the transmitter output is what it should be. You will still want to make sure the RTD and transmitter are physically appropriate for the application, i.e. still fits and responds appropriately to the system.
If you have an RTD that is functioning properly with your transmitter, and just want to duplicate the RTD, you could send the RTD in to have it measured and additional units manufactured to match its characteristics. Alternatively, if you have the input resistance values that the transmitter expects to see at various temperatures, you could send the table of resistance vs temperature values along with the physical description of the RTD, and it is very likely Minco could create parts to match.
It is not clear whether you attempted to perform a measurement of the RTD in a controlled environment like a thermal bath at 100 deg F, or whether a known "standard" device was placed next to the RTD for a comparison test to validate the temperature as being at 100 deg F when the measurement was made. It is possible that the temperature is something lower than 100 deg F and it is reflected as a lower RTD reading than would be expected at 100 deg F.
Depending on your needs, maybe all you really need to do is to recalibrate your existing RTD and transmitter to produce the correct output. Most transmitters have zero and span adjustments that allow you to make the correction. The main caution here is that you do this in comparison to known standards, preferably in controlled baths of 0 and 100 deg C. You would also want to do some validation that the RTD is stable. There could be some shunting due to low insulation resistance that causes the resistance to drop.
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