Conventionally, in England, a true current transformer is always specified by the ratio of the currents (not, as many do, by the turns ratio - so a 2000 : 1 c.t is a totally different thing to a 100 A : 50 mA c.t, even though they have nominally the same number of turns on the secondary winding).
In cases like this ‘:’ and ‘/’ mean the same thing.
That’s correct. A current transformer wants to drive a current in the secondary circuit. It will generate whatever voltage it needs and is able to, to make that current flow. If there’s nothing there for the current to flow in, a large c.t. will “flash over” - generate an electric arc - either across the surface, between the terminals, or internally and damage itself. The diode - actually a bi-directional surge suppressor - provides a current path when the voltage exceeds about 16 V (22 V in early versions).
There’s a picture here: https://learn.openenergymonitor.org/electricity-monitoring/ct-sensors/files/YhdcCTReportIss6.pdf