Many antique singing bowls from the 18th and 19th century employed a new construction style which was inherited from Southeast Asia. While earlier singing bowls bear more relation to bronze bowls from ancient Khorasan and Persia, later antiques borrowed an easier manufacturing method imported from Cambodia or Thailand. Most 18th and 19th century singing bowls were made in this newer style and many of them are completely plain. Many thin bowls especially have no engraved ornamentation. Even though we know that most singing bowls have some form of engraving, there is a whole class of bowls that are plain. We also know that antique engravings are often worn down from centuries of handling. Since many of the plain bowls are also worn thin, it is reasonable to think at least some of them were originally engraved.
The singing bowl pictured is a possible missing link proving this point. Here is a late period antique in unusually good condition which has intact engravings, similar to the large singing bowls of the same period. I think it is reasonable to assume that many late period antiques had similar engravings and that their current plain appearance is due to the engravings being worn away with age.