In 1958, when the East Los Angeles Interchange started to be built, the routes that were planned to go through it were a little different that what is there today. I-10 was originally to be State 26 and I-5 was a part of US 99. US 101 remained the same. State 60 was to be State 26. The routing of the interstates through cities wasn’t quite settled yet and many of the roads were planned to be State or US routes.
After the signs were installed, some of them were temporarily overlaid until the Pomona Freeway connections were completed. A portion of the original route planned for that freeway, State 26, was visible. While some have speculated it was actually US 60 underneath there, reverting to State 60 after the 1964 route renumbering, these plans show otherwise. How far east the State Route 26 signage would have gone does remain unknown. It is likely it would have gone at least as far as the 57, if not all the way to the 71, as US 60 was effectively terminated at the present junction of I-10 and State 71 about 1960.
For additional photos and information about the interchange, head on over to the East Los Angeles Interchange page.