Sorry for using Street View but I didn't really feel like driving 400 miles to find out for myself. This lens-less beacon (?) is clearly original to the bridge (the other side doesn't have one). Does it look like a porthole to you guys?
Looks a lot like this CH beacon (look at the signal mounting tabs(4), the door-holders(3), the bulb(straight up), and the thing on top(tab & holder) ) thats been going on ebay for a while:
There was two of these beacons on a lifting bridge here in Liverpool! I'll have to dig out my pictures. They were in poor shape, but the 'Crouse Hinds' markings could clearly be made out.