Hey Steve,
Unfortunately, most of the information out there on the forums is incorrect or misleading when it comes to T218. This article, by a Mr. George Vrechek, is mostly accurate:
http://www.sportscollectorsdigest.co...-of-the-t218s/ but is missing the Cloughan and some small details. On your master set checklist, Jack Johnson (Green) also does not exist with a Mecca Factory 30, no series notation back.
The only variations (besides brand/factory/series numbers) come in the first series.
C.D. Trubenbach has his name misspelled on the front only as "Trudenbach"
The card of S.P. Gilles spells it "Gillis" throughout the text on the variation reverse, before it was corrected. To make it more complicated, his name was properly spelled "Gillis", so American Lithography accidentally corrected the back, when they should have corrected the front.
These two cards have been rarely talked about but long known, for at least 15 years or so by some collectors.
The third variation comes on the card of Robert Cloughen. His name can be found missepelled throughout the text on the back, just like Gillis, as "Cloughan". I have owned a copy of this card for many years and have never seen another. I have never heard anyone else mention it, or report one. If anyone else knew about this card, they sure kept quiet about it!
The cards were clearly not printed in equal numbers and corrected together at once. "Trudenbach" is the most common error. It's tough, a very small minority of his cards that were printed (if I had to guess, based on what I've seen the last 15 years or so, I would say around 2% of Trubenbach's), but you can find one if you look carefully for awhile. Gillis is tougher, his card will appear for sale very, very rarely and stretches of years have gone by without a single one being publicly offered. Trudenbach has a Mecca 649 and Mecca 30 reverse (I have only seen one with a Fact. 30 back myself, I own it). Gillis is confirmed with a Factory 649 back. Some other collectors have told me they have seen it with a Factory 30 back, but I cannot find anyone who says they own one or has a scan, so it may or may not exist. The single known Cloughan has a Mecca 649 back. I am positive there are other Cloughan errors out there, but it seems the most difficult of the 3 and I would be quite surprised if this card was ever found with a Mecca 30 back. If anyone has these errors, I would love to know which ones you have, especially if you have a Cloughan!
For their great scarcity, the variations carry essentially no increased $ value, as people either don't care or don't know, and master set collectors seem to be almost non-existent for this issue (I'm over 2/3 there, maybe one day!).
In total, there are 637 confirmed cards in a T218 Master Set. This number grows to 638 or 639 if Gillis and/or Cloughan are ever confirmed with a Factory 30 back. These are the only two cards not presently confirmed that I think could possibly exist out there somewhere. I would be shocked if the master set ever grew beyond 639.
If Recurring Print Defects are your thing, McGrath has a variation on his Mecca 30 card with a brightly clear smudge in the lower left border (post 23 here:
http://www.net54baseball.com/showthr...=251642&page=3). The hobby seems inconsistent on whether recurring defects are "variations" or not, but it is recurring. 2 Blank backs are known (Rector and Handy, I own the Handy), but in my mind scrap isn't needed for a master set.
The Cloughan is attached; error is bottom middle: