A shift of +42 Da in MS indicates:
Lysine residues undergo enzymatic acetylation where an acetyl group CH3CO replaces hydrogen on epsilon amino group, neutralizing positive charge and regulating chromatin structure and metabolism. Addition of C2H2O minus H corresponds to monoisotopic mass increment of 42.0106 Da, observed as +42 Da shift in mass spectra. Detecting this modification on peptides helps identify epigenetic regulatory marks and metabolic regulation. Differentiation from trimethylation which also adds ~42 Da requires high resolution or diagnostic fragment ions, but in typical low-resolution proteomics workflows +42 Da strongly suggests acetylation event on proteins.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.