Oliver Wendell Holmes Rarity

My deceased mom, encourage stamp collecting by leaving a block of 3 Oliver Wendell Holmes uncirculated,15 cent stamps from an unperforated (top and bottom coil [roll] ) with a line (not a precanceled stamp) through the left side of the first stamp in the block. The line is part of the printed stamp and not a cancellation; the divisions between the three and all left and right sides are perforated. Looking at many photos of Holmes 15 cent stamps, I do not see this combination. She implored us to find out how rare this is? Ideas?

Welcome to the forum. Sorry about your loss.

May we see a picture (scan, ideally) of the item?

We need a photo.

Lines between certain stamps near the perforations was part of the production process where two printing plates were join. BUT what is confusing in your description is referencing a "block of 3" of coil stamps. Coil stamps normally come as a strip of 3, not a block. Sheet stamps come as blocks of three up to 100, depending on the issue.

Edit:

Oliver Wendell Holmes was also issued as a booklet stamp format. That would allow for blocks of 3 with some straight edges. Again a photo would help explain what you have for the answer you desire.

Here is a coil pair with the line I mention:

The line is a normal part of production but due to the slight shift of the perforations in your strip of three the joint line does not hit the perforation.

Value:

As much sentimental value as you may wish to assign.

Cash value, $0.45 if used as postage stamps on a letter.
Cash value if sold, it will sell as discount postage at about 25% of face value or 11-12 cents.


Edited to add: A proper line pair is as I showed above, collected as a coil pair (or more) with the line between the two center stamps. Such coil line pairs sell for a few cents more than face value to collectors of such. In production lines appeared once every about 24 stamps, I am blanking on the exact number of stamps between the lines.

Thank you.