Papa-Echo:
From all I've seen and researched, I've found the items you describe to be ceramic insulators (if you/re referring to the items molded onto the antennas that run fore and aft from bow to conning tower to stern. These were "bottle green" or dark brown in color. They were not blocks.
If you really need a block, shaped like your illustration, in a scale close to 1/72--Model Expo has some small cast metal blocks that are similar in appearance.
Hope that helps.
"Should we prosper it shall be as is our custom...by Miracle!"