looking at the pictures , the cats are off the manifold [ replaced by test pipes ] , if no issue there , i would leave it that way .
Some Photos of the "Air Pump " Yes the Cat's Have been Removed from their Original Position under the Manifolds . Note the Really Poor Quality of Air Flow Design of these Manifolds all flat with extreme 90 Degree Bends right out of the Port . looks like poor performance , Heat and Bad Fuel millage perhaps .
Manifold Design looks to be made to cram a Cat underneath it .
Some have insinuated as A " Mechanic I should Know Everything " well I have Learned from Years of Experience that a Lot of Time and Money can be saved by these Great Forums and other places by asking Questions and Getting answers from other's that have already gone " Down that particular road of repair or Up Date " and a lot can be learned . That's part of being a good informed mechanic , not to just forge ahead and buy a bunch of unneeded parts . Are Hundreds of parts on a car and lot's of different Car's and models . To Just dive right in and block something off might be a really unfortunate thing to do and cause damage .
The Manifold's Look Easy to Change but that may be deceiving related to Clearance . I have heard that the 73 and 74 Year models might work but am not sure what models they are talking like a SL Model or SEL Model and such . New Head pipes would have to be made of course .
About the Air Pump : My Experience is that Cylinder Heads have different passages inside them to accommodate these Air Pumps . Some suck out unburned Partials and pump them back into the System Yuck .
Not sure just how the 1975 benz 450 SEL Air Pump system works and was hoping perhaps someone might have learned about that already .
About Smog and the Law :
Well a 1975 Year model is" Exempt from Smog Testing " in California now and requires no testing if that answers any interested area's of questions for anyone .
I Know I can Block off the Line going in to the Cylinder head from the Smog Pump . But I don't know what that well perhaps Cause to happen because I have not done or studied such . A forum is a great place to study and you might just learn the answer you need .
Anybody actually done this ? and does the Car run poorly after doing such afterward perhaps ? .
I have read and experienced that this year model has a know rich Idle condition and certainly do not want to exacerbate and such condition perhaps .