Welcome aboard, DED. Great find!
I have nothing in my database on B. Rapp. Early Philadelphia directories show that an F. Rapp operated a turpentine manufactory. There were other Rapps there, too.
Here's what I have in my database on W.D. Rapp:
HARD TIMES ERA. WILLIAM D. RAPP WAS A SILVERSMITH AND WATCHCASE MAKER FROM 1828-50 IN NORRISTOWN AND PHILADELPHIA. HE WAS AT 256 RACE STREET IN 1837, AND 1835 APPEARS TO BE THE TIME HE ISSUED C/S's. IN 1835, HE ADVERTISED HIMSELF AS A JEWELER AT 256 SASSAFRAS. IN 1839, HE IS LISTED AS A WATCHMAKER AT 256 & 254 SASSAFRASS.
The CAST STEEL c/s could have been applied by someone other than B. Rapp. I had difficulty seeing your pic and originally thought it to be WD Rapp! (aging eyes, mine) Your B. Rapp may have been a blacksmith or maker of metal products, assuming the CAST STEEL mark is his. It's not unusual to see two or more, different counterstamps on a coin and it's generally a plus in terms of research and value.
I have recorded 24 and counting of the W.D. Rapp counterstamps on various coins; this, with but two in my collection. While 24 specimens seems like a relatively low number, it's actually a fairly "abundant" amount for silversmiths in general. For most of the early silversmiths, there typically are recorded less than a half dozen specimens. The silversmith who issued far and away the most counterstamps was "STONE & BALL" of Syracuse, NY in the 1850's.
Brunk listed five copper coins with the B. RAPP c/s in 2004, one of which was a
Half Cent; and, the latest date being 1826. It's quite likely that there are others yet to be recorded. I've not seen one as yet. It's a fair guess that this issuer was stamping coins in the late 1820's on; and, has potential to be a Hard Times issue..
Interestingly, Brunk lists a B. RAPP c/s along with an H. REES c/s on a 1798 large cent. Henry Rees was a Philadelphia blacksmith, located on Arch Street in the 1830's. Interestingly, some of Rees's pieces bear a CAST STEEL mark, too. It's quite possible that these two chaps knew one another or even worked together in Philly back then. While there are a number of Rapp chaps listed in the early Philly directories, I'm unable to spot a B. Rapp among them. There was a George Rapp, listed as a cooper. It's noteworthy that not everyone was listed in the directories, and folks were constantly seeking greener pastures back then when times were tough.
The Brunk # for W.D. Rapp is R-79, while Rulau listed it as HT-414. It's classified as a Hard Times token which makes it more desireable to token collectors of that era. While counterstamp collectors mostly seek a sharp, clear stamp, the condition and type of host coin plays a big part in valuation. Were W.D. RAPP stamped on an early Bust, silver coin, the value would be $300 on up. Large cents tended to have been the most common host coins used to circulate advertising counterstamps. I'd estimate the worth of a W.D. Rapp c/s on a low grade large cent to be in the $50-$75 range. Of course, values can soar on these at auction when two or more bidders aggressively vie for ownership. If your B. Rapp c/s can be attributed, it's value could approach that figure. Otherwise, maverick counterstamps seem to be selling nowadays in the $25-$35 range.
Summarily, B. RAPP was likely located in Philly and might well have been related to William D. A genealogy search could offer some leads to attribute the piece.
*** This post might be better suited to the exonumia section ....