Just ran across this discussion today. It seems that two stamps on each of the two plates that were used to make the 1c Nebr. overprint lacked the period at the end of the Nebr overprint. According to the Scott 2012 Specialized Catalogue of United States Stamps and Covers the plate numbers are 19338 and 19339 and the stamps affected were positions 26 and 36 of the upper right pane. Each plate produced sheets of 400 stamps separated into panes of 100 stamps so a quick calculation gives 4 stamps out of every 800 will have the missing period or 0.5% of the stamps produced. According to the website [link removed by eBay
]. there were 8,220,000 of the #669 produced from the two plates and given that 4 out of every 800 stamps don't have a period the total number of stamps that can be called a #669b are only 41,100 – a very small number. This information comes from Mike Girard of Riverdale Stamps.
I had acquired a block of 8 including two of the missing period period stamps in a collection of Kans/Nebr stamps some 40+ years ago. I just sold the block on eBay
recently. Some pictures from the block for your enjoyment.