Which Meisterstück model is a black-gold fountain pen with 135 mm length, serial number "X 61781150" and 14K nib?

With the dimensions and properties mentioned, it is a Meisterstück 144. A monochrome yellow-gold nib with longitudinally ribbed nib holder points to an older variant of the model, e.g. from the 1980s. This variant differs from the later bicolour nibs but is unambiguously to be classified as a 144.