Aye Alex,
My 1974 450 MKIII Blue/Gold, bought by me in 1976, has 18" wheels front and back. Borrani 'RECORD' rims WM2 front and WM3 rear. Front wheel the inner and outer spokes appear to be the same. BUT, THEY ARE NOT! There is a slight difference in overall length of ~5mm and the angle of the head. Inner spokes have an obtuse head angle of 95degrees, whilst the outer spokes have an acute head angle of 85degrees. These measurements are taken from the OME spokes and they are galvanised steel. If you intend to order spokes from Central Wheels, or any other supplier, I would advise that you take images of the wheels to show the lacing pattern of the spokes. You supply a pair of inner spokes and a pair of outer spokes from each wheel, clearly labelled and tied together. The rear spokes on these wheels are similar, but again, there are subtle differences in length and head angle. Also, BEFORE DISMANTLING the rear wheel, measure the distance between the outside face of the hub on the brake side and the rim outer edge and the same measurement between the outer face of the drive side of the hub and the rim outer edge. It isn't a bad thing to check the same rim offset on the front wheel as well, just in case! With the rear wheel they are different and if the wheel is not rebuilt with this offset then the rim will foul the frame on compression!
Chrome plated brass nipples and galvanised spokes are not as rigid as stainless steel spokes, but horses for courses.
I also have a 1975 350 MKIII Blue/Gold and this has a 19" WM2 Borrani front rim and a 18" WM3 Borrani rear rim on the same Oldani hubs as my 1974 450 MKIII.
As with most Ducati single parts and especially towards the end of production, if it fitted and looked the part that's what was used. Bit like 'Desmo yellow', with over 50 differing shades.
Use what you prefer and don't get in a fankle about it.
Good health, Bill