I have found that ESE is the symbol for stock in the ESCO Technologies Inc. Could it be that Chelsea Bridge owns stock with this company and decided to add this to the puzzle? Chelsea also mentioned in a previous clue about time. This is obviously a very important aspect of the stock market so who knows?. This sounds crazy but I have an idea of how ESE could end up being the letter "Q". And for once it isn't a crazy numerology!!!

OK, here it goes. It makes sense that this might fit with Chelsea's time clue because the stock market is always fluctuating so time is of the essence. So people are constantly online looking at the ups and downs of their stock. So that would mean that they are looking at the quotes. Quotes starts with "Q" and you might possibly have your letter.