Interesting find - Error stamps on Cover.

One of my professional philatelist friends sent me some rather interesting Air-India International June 1948 flight covers, which included examples of the two known varieties of Constant Errors, but this time on flown covers. I must add that I'm thrilled to have these, and this is the first time I've seen them on cover. Not sure how rare they are, since each sheet had 160 stamps (8x20), and one example of each of the two errors varieties on each sheet. My understanding is that about 3,500 sheets were printed (560,000 stamps). If this assumption is true, then there should be 3,500 of each error variety, and most of them on flown covers. I would love it if readers posted their comments on this hypothesis.

So, here is the first error, the 'Broken Tail' variety. You will notice that the center fin of the tailplane has a break in the leading edge. The Official Souvenir cover is correctly franked 14 as. which was the postal rate to Switzerland, and went on the first flight from Bombay to Geneva.

Below is a cover with an example of the 'Extra Porthole' error. This is on a Registered Air-India International Souvenir cover flown on the first flight from Bombay to London, and then onward to New York. The cover is franked with a block of four 12 a. special postage stamps issued for use on that date. The correct rate would be 24 as., and it has been overstamped by another 24 as.

Now, focus on the stamp on the bottom row on the right hand side. There, quite clearly, you will find the 'Extra Porthole' error stamp.

I hope this provides some motivation to all readers to carefully scan through their collections and look for covers with such stamps.