Plate position arithmetic

What do you know about crossover sets? I have a connective run of 10 notes from the Atlanta district. ($20's UNC) The first six are from the 1934 series, the last four are from the series 1934B series. I was under the impression that the changeover sets were, like 1928 to 1928A , or 1928A to 1928B and so forth. Any insight into this?

And on the printing process, the highest numbers are printed first, so that the lowest numbers are on top at the end of the printing. -O)

 

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The BEP's presses, then as now, use several plates in rotation (so one plate is being inked while another is printing while another is being cleaned off for the next cycle...). During the 1928 and 1934 printings, the BEP used all of its printing plates without regard to signatures and series letters: if they had some new 1934B plates and some old 1934A plates available, then they might very well have put some of each on a single press. Then that press would produce a stack of sheets that keep switching back and forth between 1934A and 1934B. And when those sheets went through the serialling operation, the result would be lots of changeover pairs--and note that half of them would be *reverse* changeovers, with the 1934A note having a higher number than the 1934B note.

In order to get changeovers that skip a series, like your 1934/1934B pair, the BEP would just need to have some old 1934 plates last long enough to end up on the same press with 1934B plates. As it happens, the lifespan of a steel printing plate is pretty long, compared to the rate at which the BEP printed currency in those days, so changeovers like this aren't terribly uncommon. In the extreme case, you can even find $1 SC changeover pairs that are 1928A/1928E!

Still, those are some pretty neat notes you've got. While changeover pairs were printed quite commonly back then, there wasn't a lot of interest in collecting them at the time (especially not in high denominations like $20's). So they can be tough to find today, just because it's fairly tough to find *any* long run of consecutive notes from the 1928 or 1934 series....

 

Numbers gave a great explanation.

The final press runs for many Series of 1934 face plates lasted into 1946, which was after the first Series of 1934B faces were sent-to-press. For many district and denomination combos, Series of 1934, 1934A, and 1934B plates were concurrently being used, with normal and skip changeovers printed in the process.

Compared to normal COPs, skip COPs are quite scarce.