Blocks.

This is coolbert:

USS IOWA once more rules the waves.

Thanks to Mike through Harry at Sharkhunters.

"USS IOWA – The huge World War Two battleship USS IOWA (BB 61) is now a museum in the port of Los Angeles, but a new USS IOWA (SSN-797) is being born . . . this new boat, the 24th in the VIRGINIA Class submarines has just had the keel laid for her construction . . . [the USS Iowa is a] Block IV boat, requiring three major overhauls during her lifetime rather than the four required of older boats and thus allowing her one more deployment."

Battleship yesterday, submarine tomorrow.

As to the term Block, i.e. Block I, Block II, Block III, etc. consider:

"[it] comes from US military procurement, where it refers to the actual items (vehicles, weapons systems, etc.) being produced and delivered. For example, if you want 10 ships of a single class built over 4 years, but the latest and greatest radar system won't be available right away, the contract might refer to two 'blocks' of ships, 4 ships in Block I with the older radar, and 6 ships in Block II with the new radar"

Older vessels of earlier blocks when undergoing refurbishment done so in a manner that latest innovations included in the process. A Block I boat meeting Block IV specifications.

coolbert.