Gaiters on forks

They push on to the same oil seal retainers. Advantage is perhaps that they avoid stone-chips to the stanchions. Disadvantages (in my opinion) is that they have to be vented or they would inflate. The breather hole allows damp air to enter so at the very least, they are a condensation trap (in NW Europe at least) and if you ride during the winter then salt water will find its way in. It's impossible to clean behind them in-situ but the short covers allow a wipe with an oily rag.

'Sensible' riders always had them but I can't see how they contribute to longevity of seals and stanchions.

I fitted them to replace the short ones when I bought my bike in the mid 80's, I put the breather holes at the rear, finally needed new stanchions and seals in 2015. That includes 20 years in SW Scotland where the rain is constant, warm and horizontal.

If you like the look of them, Richard then I think that's reason enough to fit them...but my feeling is that they aren't a maintenance-free item. Best to raise them once in a while, give everything a  dry down and a wipe over with a light oil.

I ran mine with the holes behind but the bellows action always seemed to draw in dirt and moisture regardless.

My recollection at the time is that most of the bikes sold in the UK were fitted with gaiters. I don't know if dealers / customers had any input in this.

One of the most experienced commando riders in our local club highly recommends them. His 160,000+ mile MKIII used them all it's life and he still owns it today...he claims they give the forks about 4X the life of the short hard boots.

I now use the bellows gaitors on all my bare leg commandos.

… if you plan to ride your bike rather than polish it.

I was converted, years ago, when I took the short scrapers off my Fastback and discovered the cache of oil grit lurking within.

If that doesn't convince you, consider MZ 250s, which all came with substantial and effective gaiters.  (MZs are the ultimate bike for the practical rider - the workshop manual has an appendix with drawings to make all the special tools, if you don't or can't buy them.)

... and to answer one of your original questions - no different threaded collar required. Secure gaiter to standard collar with either a cable tie or thin jubilee clip. Rode my gaitered Mk 3 850 round the dusty , gritty and sometimes plain dirt roads of Kenya for 20 years with no sign of wear on stanchions after bike came back to UK and was subsequently rebuilt. 

My MK1 850 had gaiters but I much prefer the cleaner look of the short boot gaiters. Is there a school of thought that says any moisture trapped there could cause pitting. When my Commando is not in use I wipe the stanchions with ACF 50 to deter rust (you fussy so and so you}