I don't think the 450's got sensors until 1980. My project 1977 is very original, and does not have one. And there's no point to a sensor unless it's connected to some sort of mixture control. On the SL's this involves a valve on the left side of the intake manifold and a computer in the passenger footwell.

You may have an incorrect exhaust pipe installed. But, buying and installing a generic single wire Bosch sensor, and measuring the voltage is a good way to check the mixture. Several posts on this.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe