Hi guys...I usually pay more attention to pre 1940 stamps, but this stamp has bugged me for some time.
So I did an in depth analysis to see what was there...It was probably do my training on analysis from where I worked..Or just plain flyspecking.
We all now the varieties that are associated with this stamp.
648i ( red line vertical hair line) as mine shows below....


Also 648iv (red dot to right of head)

Also 648vi ( red line below Union 1874) This one is located in the blue banner and not in the white as Unitrade describes.

There are also Smudges on some stamps as my stamp below shows.

There are also ink pulls.

There are also light/dark versions of this stamp.

There are off center printing as the 3 stamps below will show you.

NOW FOR THE INTERESTING STUFF.
Unitrade counts perfs as 12.0 x 12.5
Bottom stamp is as Unitrade says, but top one is 12.1 x 12.5, not a lot but noticeable in the image.

I took a random 100 stamps for this next analysis..Take a look how I identified other stamps..In the world//union square to the right, we have lots and lots of colour variations on these stamps.
Below is a look at just one stamp with arrows pointing to the colour varieties.

The colour differences are show in the picture below...The dark violet is a normal colour..But a far lighter violet is apparent on some stamps.

Now for the details..I took a picture of that particular box and lightened it as to be able to add identifiers as shown below.

THE CHART BELOW IS JUST A SMALL FRACTION OF THE DATA COLLECTED.
The yellow boxes are the stamps that are normal and have no colour change.
BOTTOM LINE...We know varieties exist and Unitrade does not display all known varieties for all stamps, cause they think the book would cost too much and be too big.
I spent a lot of time on just this one stamp..What do you guys think.