The date code is on the backing side of the upholstery. There is a logo, apparently the manufacturer's, "GS" then below it is the date code 10 68. I had 8 68 on prior seats. The logo and date code are repeated many times. Easy to see if you turn inside out.

Really hope someone else has encountered same issue. I really need to replace seat springs on the bottom as they are worn but don't want to proceed further until I understand what replacement foam to use.