Heating ducts...?

Question:On some heating ducts I noticed 3 numbers...they were 5 23 63... They are stamped on with black numbers. Is this the date of manufature of the ducts, or just a serial number?

Answers:
Yes, it is the date of manufacture if all the ducts are numbered identically. If it were a serial number, the numbers would be different on different ducts. It may be the installation engineer's balancing notation, but that would be hand written and not stamped.