Block to block refers to the aft boom block and top traveler block. Don't know how you get more than block to block with any blocks. Perhaps, as you suspect, it means to "super-vang." Sounds like a quick way to break a boom.
That is very odd. I thought only the boats made in the first two years, early 70's, had the number under the bow eye. That would be four digits. Plus, your number sounds like a sail number. The rest of us have a HIN, hull identification number, that is coded.