I'm not familiar with non-North American vehicles - but I would guess that the 97 C-Class is R134 not R12 since the NA C-Classes (C230/280) of the era were mandated to be R134. If it is R134 then the hi/low side fittings are different sizes and you cannot connect the gauges wrong.

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Daily Driver: 02 E430 4MATIC
In the family: '03 E500 // '04 ML500 // 64 220SE