The 280SE was offered in the US in 1977.
There should be a tag in the driver's door pillar that says the vehicle complies with US DOT requirements as well as a tag in the engine bay that says the engine complies with Federal EPA requirements and hopefully Calif EPA requirements.
Telltales of a Euro car include slim chrome bumpers, flush headlights, fabric upholstery, manual/rotary AC controls, hubcaps, metric gauges, no red segment on the portion of the taillight that wraps to the side, manual windows, '!' triangle in the trunk, large hole fuel filler, etc. Probably the most obvious is the lack of a VIN tag on the driver windshield pillar.
Lack of EGR and AIR pump are not promising. Does it at least have a CAT?
Who smogged it when you bought it?
If it's a Euro in Calif, you'd better have a red Bureau of Automotive Repair (BAR) tag in the driver door jamb or your smog check will be less than a fun ride. Best case is that they smog it, worst case is they send you to a referee. Ugh.
Start another post asking if someone with access to the EPC can run your VIN. That will tell you specifically where your car was initially delivered or at least show whether or not US and/or CA emissions equipment were installed.