On these blocks, we can see that instead of being located on the two bottom positions, they are instead found on the two right positions, which is consistent with the fact that the printing plates were rotated sideways when the sheet format was changed from 400 subjects to 600 subjects.
So it would seem that all of the plates on horizontal wove paper should have the dots on the lower positions as in the first two scans, but all plates on vertical wove paper have them on the two right positions as shown above.
However, it is not nearly so simple as this. In working with some of the blocks from the higher plates, I noticed that some of them have no position dots at all, like these two 5c blocks from plate 17:
As you can see, there are no dots at all on these, even though there should be, based on what we have seen so far. It is possible that none of the blocks from plate 17 have dots, or that maybe some do and some don't. This is a ripe topic for further detailed study.
In addition there are other differences as well. I have seen some blocks that have two dots, as well as blocks that have dots in the upper positions instead, and those that have dots in slightly different positions from those shown as follows:
Finally on this block there are two dots, one above the other, over the "A" of "Canada's". Furthermore, you can just make out a feint horizontal line under the lower position dot. What is interesting about this block is that the dots appear on the upper right position at the top, which is not seen on any of the other values.
So in conclusion, what at first appears to be a small minor detail, has become a source of intrigue. What do these position dots represent? How many different combinations are there? What plates are they found on? Are any rare?