origin of "5 seconds flat"

Does anyone know the etymology of this expression?

"He ran down the street in five seconds flat"

I found this explanation of meaning at Wordreference but would like to know where the expression comes from:

"In seconds flat" is an uncommon way of saying in no more than a few seconds.

What kind of word is flat in this instance? Is this a UK expression or used worldwide?

Sorry, I know that's 3 questions, but they're all connected.

Flat has been used as an adverb to mean ‘exactly’ since the sixteenth century. The OED records its use in the context of various kinds of measurement as being of US origin. The earliest citation in this sense is from ‘Webster’s New International Dictionary of the English Language’ in 1909.

It's originally from the US but is now used worldwide, and the earliest examples describe times in running, bicycle and horse races that are dead on to the whole second, rather than a number of seconds and fractions (typically fifths) of seconds.

This may because the second hand was exactly flat against the second mark on the watch dial, and not in between. For a modern analogy, you can compare it to an exact measure of, say, a litre of flour where the top is levelled off and there's no discrepancy.

The OED's first citation is a 1909 dictionary, but I found a 26-year antedating.

The National Republican (Washington City (D.C.), May 15, 1883) gives horse racing results:

Joe Murray, who is looked upon as a very possible winner of the handicap, went a half mile in fifty seconds flat by three watches, but pulled up seemingly tired.

The San Francisco Call (May 01, 1898) gives some times with fractions of seconds, and one in seconds flat:

He took the sticks beautifully and in the fast time of 15 4-5 seconds. ... Morgan ran the 220 hurdles in 26 seconds flat, equaling Torrey's record made on an Eastern track.