Current transformers, I suppose...

These look like the sort of CTs that are in high current circuit breakers with electronic trip devices. A three pole breaker would have three CTs, and the mounting looks like one could be added in the middle, where there is a hole for a third conductor. The round holes with square notches look like they could accommodate rectangular as well as tubular bus. This could be for a two pole breaker, which is normally the same frame as a 3 pole with the center contact assembly removed.

Most CTs have 5 amp secondaries, but the high resistance and high ratio makes me think these have 100 mA secondaries. In that case, they would have a primary current rating of 400 amps, and probably would be able to handle

10 to 15 times that before saturation (so that instantaneous currents could be properly measured). Since the CTs also supply power to the trip device, it makes sense that they would provide 20-100 mA over the normal range of operation. The trip devices usually have a backup battery or capacitor so they still operate with no current flow, but normal currents would keep it charged.

You can test these CTs by winding 10 turns of heavy wire (8 AWG or so) through the hole, and then energizing the secondary with a variac. Read the output current with a clamp-on, and see where the ratio becomes non-linear. That is the "knee", which is about the maximum these will handle.

Good luck,

Paul

Laminated iron seems to imply the laminations are standard silicon steel, and so the nominal frequency would be 50-60Hz, and the highest suggested frequency would be 400Hz.