The shell was the same but the T cars had a lot less options/ equipment in them hence they were lighter. Not special steel in those years but there was lighter gauge shells in the 1970 Model year and possibly in the 1969 model year (apart from the GTS's)
There is a suggestion that at least some 1969 body shells were "lightweight" but i have not yet seen documented proof. I am hoping this thread might draw out that type of information
The 1968 T homologation weight (FIA 577) shows 923 kg (2035 lbs)
The 1966 911S homologation (which I think that they were still using as I cant find a separate 68s one and it has attachments to it with 68 vin numbers) shows 974Kg or 2149.4 Lbs in the same spot (which is with full equipment including spare tire and tools and ready to drive but with an empty fuel tank)
As a matter of interest the 1969S homologation (FIA 608) shows 940kg and the 1969 T homologation (FIA 607) shows 974kg in the same section as I used for the earlier years and the 1970 S (FIA 3025) (Homologated especially as a lightweight version) shows 960kg hence my thoughts that there may be "lightweight shells" around in 69.