Oecolampadius uses this expression in the Toodleoo thread.
Is it standard accepted US usage or is it regarded as dialect?
My UK proofreading mind always stumbles on it and wants to correct it to ‘A couple of guesses’.

I doubt if it would be acceptable in written English in the US, though it’s common enough there in the spoken language to be considered idiomatic, if not standard.

I assume it comes from the German ein Paar, which doesn’t take the genitive, or more likely the Yiddish equivalent. I’m just guessing here, though.

I would think it could be a worn-down version of the worn-down version “a coupla guesses.”

It’s standard spoken colloquial, but not used in formal writing.