Interesting numbers on the underside of an original '74 fuel tank

I'm getting my 1973 MK1 850 Commando tank repainted and noticed a alpha/numeric stencil or stamp (C6 28 P) on the bottom side of the tank in silver paint. I believe that this tank has the original factory paint and am wondering if anyone knows what this number refers to?
And should I have the painter save this mark as a matter of authenticity/provenance?

I'm going to have Brent Budgor from Vintage Vendor do the paint and he recommended that I ask this gurus at this forum for any info.

Thanks,
Ray