R-12 can be identified by threads on the outside of the port. High and low sides are different diameters, the high side being larger. Hoses are usually color coded blue for low, and red for high.

R-134A ports are of a "quick disconnect" style, similar in function to a air compressor connection. High side is also larger diameter than the low side. Hose connections have a valve at the disconnect point. It's a rising stem valve, screw out to close, in to open. As with R-12 manifolds, blue is low, red is high.

R-12 and R134A also have different fittings on the hose connections to the manifold. R-12 is 1/4 inch flare, not sure what R-134A spec is...

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles