about 5 years ago, i purchased a 4" model 66-1 that had been originally bought by a San Francisco field office FBI agent, serial 55Kxxxx, which corresponds to 1978-1979 period per Supica/Nahas' book.....ive got original receipts that list the agents name and address, the guns model, barrel length and serial number, its from Markl supply co, pittsburgh, pa and shows a shipping address to Fed Bldg, 450 Golden Gate ave, etc.......By all accounts S&W only made 4" sq butt guns, and the box confirms this, but this gun has been converted very well to round butt, i understand that FBI agents had the option to send these to dept armorers in Quantico for conversion, the gentlemen who sold me the gun said he had it converted.......I can only find reference to model 19's being so converted, but this is its SS "cousin" the 66.....is this something anyone else has heard of?