Gold eternity ring which is stamped 901 can anyone tell you what this means?

If you are positive the ring is gold, the 901 could refer to either the stock number of the jeweler or a rather unusual gold content measurement. The number 916 or 917 indicates 22-karat gold (91.6% gold). So, 901 could be 90.1% gold.

However, if you are not positive the ring is gold, this is likely to be the quality stamp to indicate the amount of platinum used in the band. "IridPlat" or ".90Plat/Ir" is used to show the band is 90% pure platinum.