Abstract: This invention relates to compact slitting mechanism units structured so to enable a plurality of such units to be positioned in abutting, close, parallel relation, for narrow width cutting, slitting or other operations, said units including pressure chambers and knife holding and aligning mechanisms with structural features enabling knife elements and actuator cylinder mechanisms of the units to be mounted in essentially unitary axially aligned relation, and having novel means for mounting and actuating the knives and for enabling removal and replacement of any knife of any unit without disassembly of parts, in a safe convenient and simple manner, the pressure chamber-knife-holding mechanism assemblies being structured of equal widths, to facilitate side-by-side positioning of plural assemblies thereof, enabling narrow width contacting operations to be performed on surfaces to be slitted or otherwise multiply so contacted thereby.