Congratulations on your Nemrod Clipper... an iconic 60's speargun.
They are very nicely made and quite easy to get back into working order so long as the piston seals are still OK. Supplies of those dried up 20 years ago, and no-one to my knowledge makes replacements
Shaft length depends what model Clipper you have, they all used 9mm diameter shafts with a proprietary endcap... with a M7 internal thread.
Not a standard shaft diameter these days, and very heavy.
I need to measure some of mine and get back to you...
You could use a current 8mm dia hardened SS shaft but then Spear Guide #378 that fits inside #4639 Muzzle Body Front would have too much play 0.5mm per side. Could easily get a new bush turned up out of 316 Stainless to be a loose sliding fit on the 8mm shaft.
The smallest Clipper I looks very cool and 007 but doesn't have enough power to propel the spear very far at all.
The medium sized Clipper II really packs a punch when all the o-rings have been changed, the oil has been replaced with low friction synthetic 5W motorbike fork oil and the barrel pumped up to around 30 BAR... they also recoil HARD overcoming the inertia of the heavy standard shaft.
Only major issue is they are negatively buoyant after firing and drop like a rock into the sand which can then get into the barrel and score it. Also line retention using the rubber (and usually perished) #3792 Line Holder is very poor. Late model Clippers had a much improved integral line release in the lower part of the trigger guard.
More modern pneumatic spearguns like Mares float after firing and use 6mm or 8mm shafts.
Spearfishing on SCUBA was once legal in Australia in the earliest days but hasn't been for many years, so I fired my fully restored Clipper II a few times while snorkelling then went back to band guns!