Looks like it is for a 300 ohm television antenna.

Agree. 300ohm flat antenna cable for TV or FM radio.

I agree the faceplate looks like its for flat feeder but why two coax behind? In UK these would be 50ohm or most likely 75ohm. Can't work out the internal resistor/capacitor connections from the photo, but perhaps some kind of multi outlet signal distribution system???

It's probably working as a balun to match the 300 Ohm to coax or vice versa.

I've had the same type of outlet in several apartments I've lived in back in the 70's. This was before the days when cable TV was common, and the apartments usually had a master TV antenna, which was amplified, and the 75 ohm coaxial signal was run to an outlet such as these in each apartment. The balun adapted the 75 ohm signal to the balanced 300 ohms required by most TV's of the time. I'd say these were installed before TV's commonly had 75 ohm inputs (before 1980 or so). Cable TV and the need for separate runs to each customer pretty much eliminated the need for these types of outlets. The modern day equivalent is the 75 ohm jack mounted to a wall plate.

2 wires is i think in and out, but seems like for tv/radio

Beachboy's right on the money.

That's a wall plate for an older style TV set.

It has a balun as part of its construction so it can take 75 Ohm in the wall and feed 300 Ohm to the user in the room.

Back in the late 60's and early 70's i install hundreds of these. That outlet is made by Blonder-Tongue. Jerrold also made them. They were tap style. Each had a tap rating in db as to how much signal they passed from the trunk cable. They made them in 300 Ohm screw terminals or 75 Ohm F or G jack. You would put 8 or so on a run using lower loss taps to the end. This would compensate for the cable loss. Most were rated for VHF signals only, any UHF stations were converted to VHF at the Head End. Channels were received by an antenna, UHF converted to VHF, amplified, and distributed thru splitters and then to drops to which these outlets were installed. Mostly used in apt & multi-unit housing.