I remember doing that, I didn't know what the codec was so I actually started from the first number 140.01 or whatever, and kept going up until I found it, it didn't take long because it was a low number.
When MGS1 was new, the first people who figured that out were the people who had played Metal Gear 2: Solid Snake. Kojima did a similar thing in that game where a couple of characters were being held prisoner in cells and they gave you their frequencies by tapping on their cell wall. But in order to decode the taps you needed the instruction manual.
Yeah.. I was like trying everything possible while having the disc selected in items. This wouldn't have been bad if the dude didn't give you a disc after telling you to look at the CD case. That's like the biggest trap ever =(