When he said it was mostly from the 1950's I assumed he was not talking about porclein decanter figurines, but the glass decanter with designs in gold leaf - more along the lines of the Cabin Still stag decanter. You are of course right if you are talking porclein, but if they are glass decnters, he should be able to see the whiskey and if it is not cloudy, it will be drinkable. If he does not break the cork, they will seal just fine.
Schenley did a lot of glass decanter holiday bottles starting in the early 50's. There were decanters for Harper, Charter, Dant, Pepper and Schenley Reserve. Brown-Forman did a few decanters for Old Forester, and Heaven Hill and Barton did the same for Old Heaven Hill and Very Old Barton respectivly.
"Our people live almost exclusively on whiskey" - E H Taylor, Jr. 25 April 1873