For some reason by the 16th century there's a shift from the Spaulder or pauldron connecting directly to
the rerebrace, instead there starts to be a rotating collar joint at the top of the rerbrace.

Does any one know why this change becomes so popular?
By the 30 years war its ubiquitous on 3/4 cuirassier armours across europe.

When the earlyish dated example is? is this 1510 one it?

Or where it comes from?

My understanding is that until very late in the 16th century, most things we would call a "pauldron" were detachable from the upper cannon of the vambrace. Armour was modular, and the pauldrons were a module which was often left off because it was heavy and restrictive. In the 16th century some light horse wore pauldrons without vambraces.

Its likely that English spaulders in the late 14th and early 15th century were integral with the upper cannon of the vambrace. in the same period, many soldiers just wore a disc of iron to protect their shoulders from above. So both solutions already existed by the late 14th century.

Sir John Smythe the military tourist liked munitions armours to have the pauldron joined permanently to the vambrace.

How much a pauldron was detachable does seem to vary based on the armours design, the more they limit the shoulder the more likey it is for them to be removable.
That does mean you need a secondary armour, be it just mail sleeves or a second pauldron option.

There's a good video of Toby Capwell showing how Italian armours had options remove the plate components in favour of mail.

The Almain Armourers' Album shows this trend continuing in detail with its listing of exhance pieces.
But all of them also feature the rotating rerebrace, perfectly suitable in a top of the line bespoke harness but it does mean that the wear has a choice of full length arm harness or just shoulder and long gaunlets.

Viewing detachability as a negative is certainly in some military writers / buyers minds, the tassets on armour start to get there hooks an leather straps replaced with hinges riveted in place.

Yes, a few military authorities at the end of the 16th century and in the early 17th century complain that footsoldiers just toss their armour onto carts or wagons and the straps tear or the different harnesses get mixed up. I'd be careful at extrapolating that to earlier periods though!

Smythe's Instructions p. 193 claims that having the whole corselet hanging off the collar is more comfortable.

Quote:

Also I would haue them (armed pikes) to haue pouldrons of a good compasse and size, and vambraces both ioined together, and not asunder, because that the poise of the pouldrons and vambraces, hanging vpon the pinnes and springes of their collars, they doo not weigh so much, nor are not so wearisom as when they are separated; and that they weare their vambrases tied with pointes to their doublets vnder their pouldrons