Even without the historical evidence, the slogan seems an unlikely coinage without the existing idiom (surely "when it rains, Morton pours" would be more likely), and I don't know how you'd get from the marketing slogan to an idiom about misfortune.

It's a shame. Stories like this are fascinating when they turn out to be true. Like how "bucket list" entered the American vernacular so quickly and thoroughly that there are many people who believe the movie was named after a preexisting common phrase.

And quite a different interpretation.

My grandmother was the idiom queen (by the time the dementia really took hold, she was communicating in trite phrases just like 'kids these days' communicate in memes) and this was one of her favourites.

The meaning of the saying is fairly straightforward - "pours" being more voluminous than "rains", it's like saying "it never trickles but it floods". Related of course to the London bus, where you wait for ages and then 3 show up at once.

Morton's seems to take the sentiment, but change the meaning - it rains AND it pours, because the "it" changes between those two verbs. A clever play on words to change the meaning of and old idiom - maybe similar to Tesco's "Every Little Helps" that's catchy because it's similar and yet different - but as you say, definitely not the origin of the trite saying about misfortune or luck.

Sure enough here is a citation I was able find easily from 1860.

I should have known better. Can’t trust even simple things these days.

https://books.google.com/books?id=V2AEAAAAQAAJ&pg=RA4-PA169&...

and definitely considered a common saying in an several 1800s books according to Google ngram. eg:

> "Misfortunes never come alone," is an old and trite saying, and "when it rains it pours" is another phrase like unto it

https://www.google.com/search?q=%22when%20it%20rains%20it%20...