According to the "SCSW 4th Ed.", your Model 657 (s/n BBA9020) was manufactured in 1988. The Model 657 was introduced in 1986, but in 1988 the 4" bbl. was discontinued, and the Model 657-1 was released. If yours is definitely not stamped 657-1, you probably have an early 1988 657 (no dash) revolver since it is a 4". The fluted cylinder was standard, but you probably have Product Code 103950 which is a 4" bbl. and unfluted cylinder. Perhaps some Forum members with more knowledge will be along soon to help out. Regardless, you have a very nice .41 Magnum revolver...we'd appreciate seeing a few pictures!