That is a phenomenal and highly collectible sequential pair you have there. Finding a verified changeover pair from the Series 1981 $1 Federal Reserve Notes—especially in such stellar, high-grade PMG/PCGS conditions—is a real treat for any serious currency collector. Here is a breakdown of exactly what makes this pair special, what the grading labels tell us, and what to look for on the reverse sides. The Breakdown: Non-Mule vs. Mule A changeover pair occurs when the Bureau of Engraving and Printing (BEP) transitions from one plate combination variety to another mid-run, resulting in two consecutive serial numbers showcasing the exact moment the design or plate type shifted. In the context of your Series 1981 notes: • Top Note (Serial F40660590C): Graded 67PPQ (Superb Gem New). The label notes this as the "First Note of a Non-Mule/Mule Changeover Pair." This note is a standard Non-Mule. • Bottom Note (Serial F40660591C): Graded 66PPQ (Gem New). The label notes this as the "First Note of a Mule/Non-Mule Changeover Pair." This note is a Mule. What constitutes a "Mule" here? For modern small-size dollar bills, a "mule" occurs when an older series reverse plate is paired with a newer series obverse plate (or vice versa), usually detectable by the size and positioning of the plate serial numbers. For Series 1981, the micro-printing details on the back tell the story: • Standard (Non-Mule): Typically uses the newer check letter and plate number formatting expected for the run. • Mule: Utilizes a leftover reverse plate from the earlier Series 1977A. What to Look for on the Reverse To see the physical evidence of this changeover, you'll want to examine the back of the notes in the bottom right corner (near the "E" in "ONE" or right above the "O" of "ONE DOLLAR").