I have the set ( 12). In good condition they can be pricey. Some are scarcer than others. Different stamps or no stamps can be found on the backs. There are two subjects with front variations, Maloney and Fairly ( with or without dugouts showing). There are 3 unissued proofs of players not in the set, and a Brooks Robinson prototype. All very pricey. The highest priced of the 12 set subjects is the Clemente. The Maloney and Fairly are the easiest to find. The two you have are less common but easier to find than some of the others.
Here is an article from SCD on them by Olbermann, and some other articles:
http://www.sportscollectorsdigest.co...irky_1968_3-ds
http://www.sportscollectorsdigest.co...irky_1968_3-ds
http://toppsarchives.blogspot.com/20...imensions.html
http://www.sportscollectorsdaily.com...-3-d-baseball/