I've never run into these before but had a Peavey Deuce/Mace on the bench yesterday and the TL604's were used to switch the reverb and onboard phaser circuits. The amp was randomly noisy and it ended up being the phaser circuit bypass that was flapping a bit (and sounded horrible when engaged). I think TI might have made them back in the day and I suspect they were cheaper than relays and LDR's back in the 90's until they stopped making them. The originals are hard to find ($$$) and there are some interesting workarounds. I only needed one so decided to roll the dice and eBay it.
Too bad, they're fast and quiet.