Tag Heuer 1500 WD1221-K-20 Serial Number Oddity??

Just bought a Tag Heuer 1500 WD1221-K-20 in very good condition and I am loving it. Everything about it is right except for a serial number oddity. It's not engraved below the model number on the case back, but at right angles to it. The s/n format is the letter T followed by 5 digits. At first I thought the watch might have been a replica but when I searched the Internet for photos of others of this model. they all share this oddity. There doesn't appear to be any info on this. I'm trying to date the watch.

Does anyone have any history or thoughts on this.

Thanks much.