3.  UC16d had a SECOND PRINTING!!  The original UC16d was issued in 1955.  In 1957, UC16d was reprinted, with a change in the overlay!  The new, "Size 2" overlay looks pretty much like the overlay pictured above, but if you measure the length "star-to-star", you get 29 mm, and the lines are 8-to-the-inch, as before.  In addition, a 3rd new overlay size was made for the back only!!  Star-to-star measurement is 34mm, and there are only 7 lines to the inch.  The letters are a shade taller, as well, 2½mm vs. 2mm before.  NOW I know why I found it easier to read.  It was larger.

     There are 4 listed configurations:

1.  Front Size 2 down, back size 3 horizontal.
2.  Front Size 2 up, back size 3 horiz upside-down.
3.  Front size 3 horizontal, back size 2 reading down.
4.  Front size 3 horizontal upside-down, back size 2 reading up.

     Now, 1 and 2 are common.  3 and 4 are errors and scarce.  Kessler prices 1, 2, AND 3 at 35¢ and I believe this pricing of 3 is a mistake.  #4 is not priced.

     And of course, configurations 1 and 2 exist with the NO PROPELLER variety.

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     4.  ONE more bit of information!...  In 1958 the GPO started using BROWN GUM.  According to Kessler, ALL varieties of the UC16d reprint exist with Brown Gum, as well as the Clear Gum.  There is no indication of price.
     We are now ALMOST at the end of the line....

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      5.  There's still one more item to discuss....the overlay breaks on UC16d listed in UPSS as ALS4c...the FO FOREIGN & ST STATES overlay errors.
     The scan at the left speaks for itself.  In addition to the FO FOREIGN and ST STATES configs which are listed in the UPSS Envelope Catalog there are also AI AIR and M MAIL breaks.

     It wouldn't surprise me if some of you have one of these in your collection, but are unaware of it.  Here's the problem:

     The SEPARATION between letters (FO FOREIGN and ST STATES) IS NOT ALWAYS THE SAME on every ALS with that break.  Sometimes it's a bit wider, sometimes a bit narrower.  If you're looing for the GAP, IT MIGHT NOT BE THERE.  And when it's narrow, the eyes play a little trick on you -- a nasty little optical illusion -- and you don't notice the doubling of the letters, thus missing the error entirely!

     Plus, most of you don't know to also look for the AI AIR or the M MAIL clusters.  There are some lines on these overlays which DO NOT HAVE the FO FOREIGN or ST STATES groups, and a casual run across the sheet might mislead you.  However, they DO have the AI AIR or M MAIL groups.  You really have to go across the line slowly and be on the lookout for ALL 4 CLUSTERS.

     By the way....sometimes the AI AIR is so close that it reads A AIR.  The I is missing.

THAT'S JUST THE BEGINNING.  THERE'S MORE!
     I've been a dealer in postal Stationery for 33 years, during which I've found my share of these varieties.  And ALWAYS, they were on the BACK SIDE of the Air Letter Sheet.  Never on the front.  It took me 33 years to learn why!
     You see, these overlay breaks ONLY happen on the SIZE 3 overlay, which is the back side of the 1957 SECOND PRINTING.  They DO NOT OCCUR on the SIZE 1 overlay, which was used for the FIRST printing, and they DO NOT OCCUR on the SIZE 2 overlay, which is on the front of the 2nd printing.  They're only on the back side, SIZE 3.

NOW, LET'S TALK ABOUT CONFIGURATIONS.

     If you go back to section 3.... UC16d exists in 4 configurations.  I have both #1 and #2 in my inventory as Overlay Break Varieties.  If #3 or #4 exist as such, they are rare.

THEN, THERE'S THE GUM. 

     The overlay breaks, in both configs #1 and #2, exist with both CLEAR GUM and BROWN GUM...

FINALLY, THE PROPELLER? 
    
     All I can tell you at this point is that a search of my inventory has not revealed any of the overlay break ALS with the MISSING PROPELLER.  However, there is no reason to suppose that they do or do not exist.