The origin seems to lie in a switch from fraction-based pricing on financial trading markets to decimal-based pricing that took place after a U.S. government mandate in 2000.
The story is explained well in an article titled "Traders learning decimal jargon" syndicated by the Associated Press in 2000. The article starts with an interesting anecdote:
Transactions were proceeding smoothly until one trader increased his offer for 1,000 shares of Hughes Electronics with a shouted phrase. "A teenie!"
For the briefest moment, there almost was quiet in this one corner of the trading floor.
Then McDevitt responded. "You mean teenie or a cent?"
"I apologize. I meant a penny," the trader replied.
The article goes on to explain:
A teenie is trader jargon for 1/16 of a dollar. Hughes, like 100 other companies on the exchange, recently began trading in decimals with one-penny increments.
As the piece elaborates, traders had historically traded in "halves, quarters, eighths, and more recently, 16ths," and had developed their finance jargon around these fractions to simplify their language. When the market moved from speaking in fractions to decimals, their language had to evolve.
Some traders have taken to use the word "spot" to make clear they're talking in decimals, saying "35-spot-4," for example, so it's clear they mean 4 cents, not four teenies.
This change to decimal pricing was made in the year 2000, and testimony on the proposed change can be found on the website of the U.S. Securities And Exchange Committee here.
Based on the Associated Press piece in combination with the SEC record, it seems like a safe conclusion that the use of "spot" to refer to decimal points took place or became popular in the finance industry in or very close to the year 2000 to avert confusion related to switching from fraction-based pricing to decimal-based pricing.