Which wire is the C wire?
The wire labeled BL in both images, should be the C wire. W and BR activate the heat relay. G activates the blower time delay relay. PK takes 24v to the thermostat.

If you're looking for the C wire, because you want to install a fancy new WiFi thermostat.  Make sure the transformer can support the additional load, before you connect the thermostat. It doesn't look like there's a fuse on the transformer, and you wouldn't want to burn up the transformer.