As far as I understand/experienced:

• Clicking edit on a previous version of a page always sends you to the edit of the last version.
• As a consequence, there is no tool to view the "source" or thing to copy & paste a specific part of an older version.

So you are left with following options:

• by the look of a historical page, try to figure out how a person has created a block of content. Do a raw copy and re-do the layout by hand
• re-instate an older version, and do all the intermediate changes by hand again.

If this is all correct, I think it would be great to have an Read-Only mode for the SOURCE of older versions of a page, to allow easy copy pasting of specific parts into a newer version.