In the original version of the nursery rhyme, The Wise Men of Gotham, the word 'had' is used in the main clause of a sentence where it seems modern English would commonly use 'would have'. The full rhyme is:
Three wise men of Gotham,
They went to sea in a bowl,
And if the bowl had been stronger
My song had been longer.
What is the grammatical tense, and semantic sense, of the second 'had' in the rhyme?