It is very possible these came in as an estate collection, after the owner had passed on.  If so, I would hope that the builder got some satisfaction for their money. 

Of the instructions, perhaps these were all placed in a "secure location", perhaps a notebook, away and apart from the models.

     To expand on that, I'll explain with my own experiences: My brother and I when younger and more free, would go off on adventures and take pictures, pencil down track arrangements at places that looked good to model - someday. We were moved by Paul Larson and the articles and sites he used to document for publications. Many of the places, railroads no longer exist.
      Our father was older by then and recently retired, and would often ask if he could look over these and we left them with him. When younger, he had put the railroading bug into us and it seemed a nice way to return the favor. Photos and other items got stored at our fathers house and that seemed OK. Fast foreword and I get married, later my brother gets married, these items our dad would take out from time to time. Move decades foreword again to several years ago, my brother passed away, dad is quite old and passes away at 95. My mother moves to a retirement home and while my wife and I are on vacation,....my sisters get the bug to clean out the house. 
     My father had photos he had taken over the 30s, 40s, 50s, of freight cars, many scenes like Union station in Chicago, or the C&NW lakefront Milwaukee station, much interurban stuff of the Chicago-Milwaukee area, Denver, Detroit, points east, and elsewhere. Much stuff get tossed including guess what of my brother and mine. 
     It did not mean anything to my sisters nor the people who assisted them. Mostly all tossed. 

     For people who have been through these events, the above will remind them of times they may not care to revisit.     There is a lesson here for those who care to see it.   

 At least the kits in Pasadena got saved. And are bringing happiness to others.                                                                                                   James Dick  - Roseville, MN