Pretty sure no Disco ever got internal routing while I was there ('til '94), though I'd be happy to be corrected. (1) I didn't make them myself and (2) that was over 30 years and many fried synapses ago.

The Impulse was the first to get internal, probably around '87. Bill might remember, or his partner Bob Freeman. I made Bob a custom in '86 that was sort of a prototype of the Impulse, and it had 3 housing guides on top of the TT, as did the first few batches of Impulses. Certainly by some point in '87 they were internal. Here's an '87 Impulse with internal in Freeman's Flickr: https://flic.kr/s/aHsiNRC6jL

BTW if you don't know Freeman's Flickr, it's an absolute Aladdin's cave of wonderous bikes, check it out when you have some hours to spare. https://www.flickr.com/photos/8379107@N03/albums/