Thanks for the compliment. Yes theses railings are custom but as the wood railing people point out in the comment above there are many manufacturers that offer cable railing parts and pieces. Hayes and Feeney are just a couple.
We incorporated solid plates at the corners in this house as just one way to handle the turning of corners with cable rails. If you have custom verticals made to receive cable rails, I would suggest using aluminum or stainless steel for any fabrications outside, and inside you could use standard steel to keep from creating a rust/maintenance issue down the road.
Mountain Laurel Handrail
GMK Architecture Inc