What are en-route VHF marker beacons (75 MHz)?
En-route marker beacons are close relatives of the outer/middle/inner markers used on ILS approaches. Airway markers use the same audio frequency as inner markers, and some older equipment has indicators for airway/outer/middle. Older versions of the FAA AIM discuss marker beacons, but later versions omit them.
From Wikipedia:

From the 1930s until the 1950s, markers were used extensively along airways to provide an indication of an aircraft's specific position along the route, but from the 1960s they have become increasingly limited to ILS approach installations. They are now very gradually being phased out of service, especially in more developed parts of the world, as GPS and other technologies have made marker beacons increasingly obsolete.
The airway marker was used to indicate reporting points along the centerline of now obsolete &quot;Red&quot; airways
They are only used on a few instrument approaches anymore, to mark positions along the inbound course, when there is only one marker present

The example cited on Wikipedia (KSEE) used to have a marker as part of the LOC-D approach, but it appears that this was replaced with the YAGSU waypoint (and more recently, the DEBEY waypoint). Another example is KSLE where the ARTTY intersection on the LOC BC 13 approach used to have a marker but no longer does. I haven't been able to find any current procedures still using one.