What type of mortar should I use to repair masonry joints on a building exterior?
Sounds like a pretty standard repointing job, other than the location - rake, chisel (or &quot;grout-saw&quot;) out all the loose/deteriorated mortar to some convenient depth, wet the joints, pack in new mortar (keep it damp so that it cures, rather than drying out) and tool the surface. You'll find more information in general on &quot;repointing brick&quot; but it's essentially the same other than your joints may be a bit wider than a typical brick joint mortar-line.
1970's era construction is probably normal modern Portland cement based mortar. i.e. what your masonry supplier will have as mortar mix, not some weird old lime-putty stuff you might need for historical preservation, where using the modern stuff will cause problems since the wall was built with the old-style stuff.