Glad to hear that 1604 fixed your recording failures!
Both 1511-EPGFix and 1604 are very new and we haven't had much experience with either. The big knock I had against 1604 was that if you pressed CC while playing a recording, even accidentally, the box would lock up. The other bugs I found were all pretty minor. But if 1511-EPGFix won't record reliably, that's more serious than the CC thing. (It would be nice if iView had a firmware version without either problem.)
I did download 1309 and extract it from the .rar. It turned out to be simply V1 for the 3500s with the channel 3/4 switch. I compared both usb_upgrade_all_flash.bin files and they were identical. I'm not even sure why 1309 is there, since the very same firmware (minus the .rar envelope) is available further down the page (along with V2a and V3 for the same boxes).