Link to the spec sheets?
Likely (guessing) one may be "hardened" or may be selected for lowest noise.
The other possibilities include gain and bias...
W3JN seems to know a bunch about these Racal boxes...
Edit:
Nominally it is a zener protected switch.
Devices like this have been known to be used as linear devices...
Looking at the pdf, seems like the 5000 handles more volts.
Also there are two packages, the plastic and the ceramic.
Looks interchangeable to me.
https://www.universalsemiconductor.com/SD5000-5002.pdf _-_-