A saver bus ticket was a sort of carnet - you bought 6 tickets for the price of 5 or something like that. You tore off a ticket and handed it to the driver, keeping the stub in case of inspection. The serial number would match up with the part kept by the driver
They went off sale in 2007 or 8 (yes...) but the TfL site says you can still use them if you have any left. For how long - pass
People do still obtain saver tickets even though they're not on general sale.
When they were first on sale they worked out as 65p each when a cash bus fare was £1.