Hey Webb -
I'm not sure about TR6's, but the TR3's have similar tags made of brass, and located on the bulkhead above and behind the battery. According to Bill Piggott's book Original Triumph TR, one plate has a number which is near, but slightly higher than, the commission number, and is thought to have been allocated to the complete, trimmed body. The other number appears to have no connection to the commission number and may have been allocated to the bare bodyshell, which was supplied by the manufacturer, a company called Mulliners.
In typical fashion regarding these cars, Piggott goes on to say, "However, little can be learned from either body number, although posssibly in the future some more information may materialize."
You might want to invest in his book on TR's - some great information in there. And now he's published revised versions which treat them separately (at least the TR3 and 4), and included more details and pictures.
Mickey