You are correct Jonzobot.  Any and all gains on the investment(s) purchased with overcontribution would be deemed the penalty amount.  

What some might have pointed out is that I can always accumulate approximately $12,000 worth of shares now in a taxable account and then in January transfer the shares i.e. $6,000 worth of shares each.  Any gains on them would be triggered in 2021 therefore no taxes payable for 16 months.  And any loss would be non-deductible according superficial loss rules.  

Meh, I'll take my chances and wait for the new year.  If tax loss selling offers opportunity then I'll change my mind. :)