Some videos of what the Route 15 corridor in San Diego was like between I-5 and SR 163 (US 395) in the 1970s:
ROUTE 15 NORTH
Wabash Boulevard: https://youtu.be/EUdLnhtA4zk
40th Street: https://youtu.be/dRUb_MSxsO4
Ward Road: https://youtu.be/Y0SXWRznVnU
Murphy Canyon Road: https://youtu.be/KZMeCtdUDOw
ROUTE 15 SOUTH
Murphy Canyon Road: https://youtu.be/QzoDP5ZYFBU
Ward Road: https://youtu.be/80E7_lzbwGc
40th Street: https://youtu.be/xXKmWx6rHWg
Wabash Boulevard https://youtu.be/BRRMIo6oUPo
More San Diego city road videos are available
https://www.youtube.com/channel/UCn1v1JAjFEEvqMi-VHpee5w
It's too bad they seem to have captured them at such low resolution. They're basically repeated stills, so a higher quality scan would probably pull out a decent level of detail.
I see a good number of at grade intersections on I-15 (Murphy Rd)! I also noticed a lack of shoulder stripes on the freeway as well.
SR 15 and I-15 were well-marked on the roadway segments during that timeframe. The I-15 portion seemed to have some at-grade intersections; I wasn't sure if I-15 was signed on the roadway prior to it reaching full freeway standards, so the video answers that question. Thank you for posting!