A mystery indeed. The Model 60 is a stainless gun, so wouldn't be "chromed" though it could have been polished to look like it (while I suppose it would be possible to have a gun chrome plated, nickel is the material usually used - not on stainless, of course, and S&W doesn't chrome plate guns at the factory). Modern S&W serial numbers are of the form AAAXXXX, where the "A"s are letters and the "X"s are numerals, so "WBLP" is not a correct prefix and although they used to use various single-letter prefixes over the years, "P" is not one of them.
S&W made many special runs of various models , usually for law enforcement organizations, and stamped them to indicate such, e.g., "CRPD" for the Conrail Police Dept., "PXPD" for the Phoenix (AZ) Police Dept., etc.), but these stampings were on the side and not incorporated into the serial number. There is no mention of a "WBLP" special run in SCSW.
"BLP15XX" would be a valid serial number, from around 1992-93, but that means the "W" would have been added. I would think it would be relatively easy to determine if that's the case. I'd also think that BATFE would consider that to be defacing the serial number, with all the potential pain that could bring to the owner, regardless of who did the defacing.
The Model 60 was introduced nearly a decade after S&W started assigning numbers rather than names to its guns, so the model number should be clearly stamped inside the yoke cutout.