The first series was released with three different style backs: white, tan, and Ludlow. The Ludlows have a picture of a camel on the back, and can be found with black or red ink.. The tan and white backs are blank. The tan and Ludlow backs are more rare and are very hard to find. Band-Ache, Lavirus, Mutt's, and Paul Maul were short printed and are therefore more difficult to find than the other stickers in the series.
Many people think they have tan back first series stickers when they really don't. Most of these people have stickers from 2 spinoff sets that were released later on - the Wonder Bread sets. It is easy to distinguish some of the Wonder Bread stickers from true first series tans because they have a different copyright. For more details, check out the list of miscellaneous sets. Unfortunately, three of the designs (Gadzooka, Tied, and Lavirus) can be found in one of the Wonder Bread sets with a copyright identical to that of a true first series tan back. This means that they are not distinguishable. I think that there may be some subtle differences that could be used to tell them apart, but I'm not sure since I don't have any real first series tan backs to compare them to.
*Update* it turns out that the die-cut on the Lavirus stickers are different. The true tan backs have a rounded die-cut at the top, and the Wonder Bread version has a sharp cornered die-cut at the top. Thanks to Greg Grant for noticing this.
The first series stickers can also be found with fronts made out of a cloth like material. The cloth stickers are very fragile and tend to detach from the sticker backing if not treated with TLC. They are difficult to find in mint condition. If you want to ship these stickers to somebody, don't put them in contact with any plastic! Wrap them in wax paper first. If you put them into plastic sheets, don't expect to easily take them out a year later. You'll have to use a butter knife to carefully separate the plastic from the sticker. The glue seeps through the cloth and sticks to the plastic better than the paper backing. Your best bet is to keep them in a cardboard box separated by wax paper if you want to keep them in nice condition.
The checklists for the cloth series are almost identical to the regular first series checklists . The cloth sticker checklists have just one line of writing at the bottom (with large letters). The more common first series checklists have two lines of writing at the bottom (with smaller letters). Mrs. Klean is misspelled (Mrs. Kleen) on the clock sticker checklists
All of the first series designs were originally part of the die-cut series from 1967.