The measurement should be from the flat portion of spring cup(where the spring sits) to the underside of the retainer.......not from the head to the retainer(unless there are no longer any shims or cups being used......which would be its own problem).

Edit: I just looked at the pic of your heads without the rockers again, up close.
The Edelbrock spring cups have been removed, and it looks like I can make out the edge of what is likely an inner spring seat. The inner spring seats are usually .060 thick, so if you measured down to the head itself, and there is indeed a spring seat there....... you can basically take .060 off that measurement you got.

And if that's the case....... and you actually want to be in that 1.880 installed height range......you'll need the +.050 locks.