Which brand & model is this 50 watter? It replaced a GE 100watt FCO HPS. Thanks for your expert-ease!

That is an American Electric Lighting Autobahn Series ATBMic.

That was quick, thanks!

Interesting, with "Field Adjustable Output" module:

The Field Adjustable Output (FAO) module
is an onboard device that adjusts the light
output and input wattage to meet site
specific requirements, allowing a single
fixture configuration to be flexibly applied
in many different applications.

I thought the former head was a GE, but now I don't think so. The nice ribbed reflector behind the lamp is different. What's the HPS model?

The old HPS head was a Cooper OVH.