We can recreate early Saka cataphract cavalry pretty clearly based on a few sources.
The first is, as you have posted, the Khumbuz-tepe fragment, 4th - 3rd or 3rd - 2nd C. BC, depending on whose dating you follow, which is the only evidence we have for horse armour before the Khalchayan relief (1st c. BC - 1st C. AD)
The next major one is a golden bracer from the Siberian collection of Peter I dating to the 3rd C. BC. This is the only object from the centuries BC, and perhaps even antiquity, which depicts more than one cataphract that I know of. It shows a battle scene with three cataphracts pursuing and defeating four other cataphracts. Two of the cataphracts fire bows at the others, while a third spears a retreating enemy in the back. All wear cuirasses with long skirts reaching to around the knees made of large quadrangular plates and high collars as well as segmented arm and leg armour. All wear what seem to be simple Kuban-style helmets and ride unarmoured horses.
The next is a bronze figurine from Talas-Tal and dating to 4th - 2nd C. BC (this is #30 on that CAIS link the posted in the original post). It is of a man wearing the same kind of armour again, with a skirt reaching to around the knees made of three rows of plates, a high collar, and a simple Kuban-style helmet.
Then there is the actual find of such iron armour from Chirik-Rabat near the Jaxartes (Syr Darya) dating to the 4th C. BC. This included rectangular plates (7.5 x 9 cm) as well as a larger fragment of a plate measuring 9 x 12, dozens of curved plates from segmented limb armour, and a straight sword, which was close to a metre in length.
Finally, there are the coins of the Indo-Kushan kings Spilarises and Azes (IIRC), c. the 1st C. BC, and the sculpture of Khalchayan, which dates to the 1st C. BC or 1st C. AD. Both of these show the warriors in armour almost exactly like that seen above.
All of this evidence is strikingly consistent: the warriors wear cuirasses made of large quadrangular plates which include long armoured skirts and high collars; segmented arm and leg protection; and simple Kuban style helmets. It seems likely, based on this evidence, that most Saka cataphracts rode unarmoured horses. The horse armour, based on the Khumbuz-tepe fragments and the Khalchayan sculpture, is a trapper reaching only to the level of about the belly and which includes a cover for the neck and face.
The interpretation of the Khumbuz-tepe fragment as showing parapleuridia is almost certainly incorrect, as comparison with the other pieces of evidence above shows that this was simply the long armoured skirt of the rider.
It seems very unlikely that the full cataphracts would carry shields, and I suspect that the man shot during the Jaxartes battle was probably a lighter cavalryman. Standard armament seems to have been the kontos, bow and gorytos, and long sword.
Quote:The oblong Pazyryk shield, above, is tiny - 28cms by 36 cms and is thought to be a waggon shield, but that is supposition. It could be mounted on a shoulder I suspect but there is no evidence on it to support that style of fastening that I could see when I looked at it.
Numerous shields were found in the original Pazyryk tombs, and others have since been found in other Altai excavations, including quite large ones from tombs of warriors with other weapons. They commonly seem to have leather armstraps to be carried with, though, and so I also doubt that they were strapped to the back.