Leather '68s have appeared a few times over on the 1st Gen Firebird site.
68 400 HO /w A/C, LEATHER, clock - an ad at an Alberta Canada classified ad website.
https://firstgenfirebird.org/forums/...tml#Post279860
leather interior - A Texas owner of a '68
https://firstgenfirebird.org/forums/...tml#Post214493
68 Firebird Autumn Bronze out of the booth - "68 Firebird 400 Convt 33 option car:tilt, cruise, AC, 281 trim code leather interior, PDB,PS,P top,headrest, rear antena, Ralley 2's, TH 400, Speedminder speedo buzzer, clock, tinted windows, floor mats, remote mirror, door edge guards, remot decklid release, trunk lamp, saftey track rear, am fm radio, visor vanity mirror, custom seatbelts, console,heavy duty air cleaner, exhaust extensions, tinted glass- all. All factory installed total price $5121.18 !!!"
https://firstgenfirebird.org/forums/...tml#Post127444
The best source of 1st Gen history I have found, "The Definitive Firebird and Trans Am Guide 1967- 1969 by Rocky Rotella", indicates, "No known production numbers............." and "..... quite a few have been documented."