I am unable to tell the difference between 1033 and 1033a (Silkote paper) without using an ultraviolet light. The 1033a does have a brighter appearance to the naked eye, but it is very slight. Also, the 1033a is said to have a smoother feel, but I cannot definitively discern it and don't suggest testing for this with a mint stamp!
The 1033a is a nice "glamour" stamp that some collectors want in their collections. In F-VF, you can buy one for around $300. As explained below, nicely centered stamps (PSE 95-98) are in short supply (37 have been graded 95 and 98).
The Silkote papers went on sale in the Westbrook and Cumberland Mills, Maine post offices on December 17, 1954. A total of 125 press sheets (500 panes) were printed by the U.S. Bureau of Engraving and Printing in the late fall of 1954. The stamps were placed on sale in Westbrook, Maine and Cumberland, Maine.
The paper was developed by the S. D. Warren Co. (Westbrook, Maine) in an attempt to overcome issues related to the extreme dampening necessary when printing on the then-current sulphite paper known as "Oxford;" the Silkote paper had a special surface that made it unnecessary for extreme dampness to be applied to the paper in the printing process. The result is a sharper, clearer impression. Because the Silkote paper did not require the extreme dampness that the "Oxford" paper did, there was less shrinkage, causing the sheets printed on this experimental paper to be somewhat misaligned on the perforating machines which were set to account for the shrinkage experienced with the "Oxford" paper. Consequently, the stamps from the Silkote sheets tended to be relatively poorly centered, with but few rare exceptions. The paper contained calcium carbonate.
The stamps were released at the post offices with no notification of the paper use, but several were purchased by employees of the S. D. Warren Co. Siegel Auctions estimate that perhaps seven or eight panes are known and several have been broken. If a pane is broken down, the market becomes (temporarily?) saturated and prices for ungraded or lower graded examples (90 or less) will likely fall unless demand increases.
In 1954 you could mail a Christmas card for two cents if you left the envelope unsealed and just tucked in the flap. With no notice to the public, that's probably the way most all of them were used.
The 1033a is the stamp with the selvedge. It is graded 95.