We have a few Parker 45 italic nibs in steel complete with feed and collar in two widths. Measuring the thickest part of the italic nib's writing line, these widths are approx 0.9mm and 1.3mm. The narrow lines for both nibs are approx 0.4mm. The collars on all the italic nibs are imprinted 'D'. These are all UK nibs originally from the Parker factory at Newhaven.
Fitting is easy - you simply unscrew the old nib/feed/collar from the nib shell and replace it with the new nib/feed/collar. If the old nib doesn't want to unscrew, it's probably glued in place by old dried ink. If so, you just need to soak the old nib unit in some cold water for a few hours to loosen up the ink
These nibs are all unused ex-factory stock