The 666 and 666R were part of EV's family of "Variable D" mics, developed in the late 1950s. The best known of these is the 664, which was the definitive PA mic before the SM-58 became so popular.
The 666 also reflects EV's pattern of quality selecting capsules and building two models on "general" capsules and "selected" capsules. So the 666 is the selected capsule version of the 664. Similar selecting leads the RE-10 / RE-15, and RE-11 / RE-16.
The "real" selected 664 is the 665, with the 666 being the 'best of the best'.
These mics (664/665/666) all use multiple rear entrances in an effort to reduce proximity effect. So, if you look at your 664s, you see the grill, then openings on the sides of the head (the openings changed a bit during production, as an aesthetic, but not acoustic decision). Then on the back end of the "spine" there is a hole, which is the last rear opening.
Unfortunately these mics have foam inside the front grill, inside the side entrances, and inside the spine. This soaks up any available contamination (particulary metal particles - since the magnets are very strong). EV no longer sources clean foam, and has no published (that I know of) protocols for cleaning the diaphragms. This is important because the strong magnets of high quality dynamic mics will attract every tiny chip from mic stand threads, bits on the floor (where too many mics get laid), etc. Even some of the particles in cigarette smoke are magnetic, and are attracted to the diaphragms.
I haven't used a 664 in decades, except as film props. Back in their day, they were considered a very good quality, and very durable PA mic. Many small market stations used them for announce mics. Because of their distributed openings, they don't have the proximity effect of the bidirectional ribbons, like 77DX or BX-44 which were still popular in the '50s. This wasn't seen as an announce problem, as shown by the wide acceptance of the later RE-20, which is more of a "continuously variable D" design.
The 666s were the standard 'go to' workhorses for news announce, live performance (look at the cover on the Jefferson Airplane's FIRST album, the one with Toley Anderson as the singer, before Grace Slick), and even as inexpensive boom mics (back in the mid '60s, a 666 was only about $160 list). The 666R stands for "rising response". Actually, it was a slow low end roll off, rather than a rising high end.
For a late '50s - early '60s design, the mics are pretty flat. As dynamics, they have the frequency response and transient response limits one would expect. If you have them, try them out. Particularly for some type of 'event' or theme gigs, they offer interesting visual appeal, with no necessary loss of PA quality.
The mics were, and are considered great drum mics.
I only have five of these. One is a factory prototype, shipped to Hollywood for evaluation. It has a rougher case design, before EV settled on the final version of the spine.
The same "variable D" design was used by AKG in its "2 way" mics, of which the D-202e is the best known. The AKG mics actually use two mic elements (the EVs did not), but the multiple rear entries were used to reduce proximity effect.
One constant problem with both the 666 and the 664 is their connectors. The 664 has an Amphenol MC-4 connector. Try to find those in your local Radio Shack! The 666 uses the Cannon UA-3 connector. Try to find those anywhere! BUT, the 665, which uses the same case and stand mount as the 664, uses a standard XLR-3. So it may be possible to adapt the 664 (I've never tried this, so no guarantees). No such luck with the 666.
If you get a 666, make sure that it comes with a UA-3 cable connector. The best thing is to make up pigtails (about 6' long) which have the UA on one end and an XLR on the other. Make them about 6' long so the change to XLR comes at the bottom of the stand, or otherwise out of camera view.
Cordially,
Tom