If yours is the same as my ‘Warco’ badged machine, with the LH end of the pressure-roller working in an open-ended recess in the casting….

In front of the LH thickness-adjusting screw (on top of the frame casting) is a 12mm diameter, vertical hole…

That accommodates a steel pin that can rotate freely through 90º in the casting, constrained from further rotation and retained in the casting by a socket-head screw.

The pin is 12mm dia X 65mm long, with a 32mm long flat cut to half-diameter, starting 6 mm up from the lower end.

The M6 X 12mm Allen screw is 16mm down from the top end, its axis at right-angles to the flat, and the head is above the flat when assembled.

The dimensions are not especially critical_ verify them on your own machine.

Assembled, the screw head occupies a slot in the face of the casting, and by it the pin can be rotated from holding the top roller in the frame, to bringing the flat round so the roller can be swung out. The head is on the same side as the flat, and points towards you when the pin is in the locking position.

The machine would work with a plain pin long enough to protrude above the casting so you can pull it out to release the roller, but not so long it makes using the thickness adjuster screw awkward. You could make it just long enough to put a roll-pin or a wire ring through to act as a simple handle for withdrawing the pin.

he screw-hole is shown plain here in this quick Alibre image but that’s ‘cos I can’t draw screw-threads! It is drilled and tapped M6.