I must admit that I have never really been a big fan of slug plate whiskeys. They just do not have the beauty, and overall appeal that full face embossed bottles exhibit.
Some of the earlier slug plate fifths do have some attractive characteristics, with some having good crudity, and color. Their main appeal however is rarity, and heritage.
Here are two "slug plate" fifths. The R.T. Carroll is rare with 7 known examples, and the Renz's is almost as tough, with about 12 or so in collections. I like the thick end primitive embossing with the "curved Rs". There are some slug plate whiskeys that are unique to maybe 2 or 3 known. You have to wonder just how much more expensive it was to have a more elaborate container than these relatively plain bottles blown. The fact that most slug plates are very rare attests to the likelihood that there were just not as many produced in the first place, so they must have been ordered with only cost as a concern. Otherwise the cheaper bottle should be much more numerous.