The Floyd Rose Pro Tremolo System, made in Germany, is a more low profile version of the Floyd Rose Original which features narrower string spacing; the Original Floyd Rose Tremolo string spacing is .420 inches and the Floyd Rose Pro string spacing is .400 inches.
The earliest units were made by Gary Kahler of American Precision Metalworks (APM), were made of bent steel, and sometimes has a “droop” on one side due to the fabrication process. “APM” is also stamped on the bottom of them. APM was able to build them for Floyd due to a lawsuit Gary Kahler won (which also resulted him Kahler being allowed to create the Kahler Steeler, Killer, and Spyder tremolos).
The Pro units were installed on later model Kramers. Most notably, they can be seen on Nightswan II’s, Pro-Axe guitars, and some Showsters. They differ from the standard units in that the intonation allen screws are in front of the fine tuners as opposed to the OFR which has them behind the whale tail. Also, the “whale tail” is at an angle versus its OFR relative making the trem a lower profile for players who rest their hands on the unit while playing. The spring block that extends down into the body is shorter as well, keeping the block at the right depth for recessed routed trems. Generally, they work the same, and stay in tune under the most rigorous “whammy” conditions. They cannot be used on traditional OFR “non recessed” routes. (Taken from vintagekramer.com)