Drakewatcher manuscripts are the scrolls you learn to unlock dragon customizations.

AK itemtest:
ItemTest: AlreadyKnown 1.65 / 10.2.5
Item: [Renewed Proto-Drake: Steel and Yellow Armor] - Regions: 6/54 - Known: false
ItemLink: |cff0070dd|Hitem:197352::::::::70:250::13:1:3524::::::|h[Renewed Proto-Drake: Steel and Yellow Armor]|h|r
3 FontString GameTooltipTextLeft1 Renewed Proto-Drake: Steel and Yellow Armor
5 FontString GameTooltipTextLeft2 Drakewatcher Manuscript
37 FontString GameTooltipTextLeft3 Binds when picked up
39 FontString GameTooltipTextLeft4 Use: Unlocks this customization option for the Renewed Proto-Drake at the Rostrum of Transformation.
41 FontString GameTooltipTextLeft5 Auction
42 FontString GameTooltipTextRight5 Cannot Auction

Took it off the AH, since I didn't have any in the inventory.

Here's a Wowhead link to the same item:
https://www.wowhead.com/item=197352/renewed-proto-drake-steel-and-yellow-armor

I can provide more if necessary.

I've been trying to research this on and off when I have free time, but so far I haven't been able to

• Find any related APIs when searching the Warcraft Wiki.
• Find the customization UI while browsing FrameXML files for any references to it (I haven't even managed to figure out the name of the actual UI Frame I'm trying to find).
• Find the customization UI while playing on the PTR (I don't have a max level character on the PTR).

I'm unsure how to continue with my search other than trying to motivate myself to level one of my characters on PTR high enough to find the UI in game and continue from there.

I'll check if I can find some kind of API to check against these when I have free time to tinker on these.