I'm going by the numbers on the sprue here (and from what I see):
The right arm seems to be part number 5 on the lower left and it looks like a regular arm (just with a specific pose). You can see the cape at the top (part 3), and on the top right is part number 4 which looks like the armour front bit where the arm (5) plugs into and which itself plugs into the cape (comparable to the back part of regular power armour), with the tiny shield (9, bottom right, under the backpack) to cover it. Here's a view of the guy from the front:
https://www.games-workshop.com/resources/catalog/product/600x620/99070101011_DarkAngelsInterrogatorChaplain01.jpg
The crozius arcanum looks a bit
DA-ish (winged sword motive at the "hitting enemies with it" part) but the rest looks regular. It's the same as the front view but instead of the back of the skull you get its face. The arm cover/shield has a motive of a skull "biting" into a key and that might have some
DA relevance (Inner Circle -> keys/secrets behind locked doors) but can be filed or explained away.
The right hand power fist again has a sword/key motive (leave it or file away) and the winged sword shoulder pad (that looks replaceable).