Scott Specialized Catalog Plate Block Nomenclature.

I have been unable to decipher the Plate Block descriptions in my Scott Specialized Catalog.

I believe:
#C90 - P# block of 4 - = the plate block contains 4 stamps.
#C98 - P3 block of 12, 6# - = the plate block contains 12 stamps and 6 plate numbers.

What I don't understand:
#C129 - P# block of 4, 4#+A - What does the +A mean?
#C132 - P# block of 4, 4#+S - What does the +S mean?
#C135 - P# block of 4, 4#+B - What does the +B mean?
#C145 - P# block of 4, 5#+V - What does the +V mean?
#C148 - P# block of 4, 5#+P - What does the +P mean?

There may be others that I haven't come across yet. Where in the catalog is this explained?

Thanks in advance for any help you can provide.
Regards,
Tim

Not having the catalog in front of me ... I went to ebay to find illustrations of typical plate blocks of some of these.
For example C129 has an "A" in in front of the 4 digits, C148 has a "P" before 5 digits, etc.
Bottom line, ebay *can* be a useful reference tool.

John Becker nailed it. BUT if that does not answer your question, the individual letter can also correlate to the entity with produced the item.

Examples:

A -- American Banknote Company
B -- BEP (Bureau of Engraving and Printing)
etc.

No letter (sometimes B) = BEP (Bureau of Engraving and Printing)
A = American Bank Note Company
B = Banknote Corporation of America
C = CCI Corporation
P = Ashton Potter
S = Stamp Venturers / Sennett Security Products
V = Avery Dennison

I will add the above is only true for the period the BEP was printing US stamps.

I do not believe the BEP production overlaps with Banknote Corporation of America.

It would seem to be more helpful if Scott listed these as e.g. A+4# rather than 4#+A, as the letter is a prefix in the plate number.

Other letters that you may encounter.
D = Dittler Brothers
G = Guilford Gravure
M = 3M

I'm not aware of any BEP printed stamps with a B prefix. But there are a few BEP printed coil stamps that have an A or B at the end of the plate number. See Scott 2915A, 3281, and 3632 for examples. Rather than identifying the printer, this indicates that a set of plate numbers were reused on a new set of plates, i.e. stamps with a plate number of 11111A are printed from different plates than those with plate number 11111. The BEP did this once they had reached number 9 on the plate numbers.

"It would seem to be more helpful if Scott listed these as e.g. A+4# rather than 4#+A, as the letter is a prefix in the plate number."

Agree

Thanks for all the positive responses.
Tim