DIFFERENCES AND FEATURES
First off: no microphone preamps. You mastering engineers certainly don't need those. The mic pre and input select switch are removed and the switches become dedicated "mode" switches one each for the ELOP and the FET limiters.
 
1) The ELOP limiter gets more Ratios - 10:1, 5:1, 3:1, 2:1 and something new called AutoHF which starts off as a 1.5:1 ratio and increases gradually to 10:1 for highs. This is something like a de-esser, and of course can work with the ELOP SC toggle at 200Hz for even more effect.
2) The FET limiters get new modes that are 50%, NORM, LP Lim, BOTH, and CLIP. 
3) The LEFT blue erstwhile-"Limit" button becomes a master stereo Bypass switch here in the Mastering Version. When it is not engaged, it becomes a true hard-wire bypass (This was not possible on the mic preamp regular version obviously).
4) The right Bypass button becomes the Limiter Bypass (tubes & transformers still active).
5) The what-was-the-Phantom toggle on the back of the unit becomes the Unbalanced 1/4" jack input select in this new mastering version.
6) There are no pots on the faceplate of the Mastering Version of the SLAM! All of the knobs in the Mastering Version are discrete detented switches in mostly 1 dB steps, with some appropriate 1/2 dB and 2 dB steps, right where they should be.
More than that, we really like the way it works and sounds and we generally prefer it to the non-mastering version for stereo tracks. There is a lot less fiddling involved to maintain L/R balances and the new modes are subtle but fun. It is easy to make just about anything sound better.
 
SPECIFICATIONS - MASTERING VERSION
Input Tubes: 2 x 12AT7A NOS GE specially selected by Manley Labs for lo-noise and stable bias
Output Tubes: 2 x 12BH7 (or 6414W NOS USA) dual triodes
Balanced XLR Line I/O: MANLEY IRON® transformer-coupled Balanced Inputs and Outputs
Gain: 20dB max Limiter Gain
Input Impedance balanced input XLR combo jack: 2 kOhms
Input Impedance unbalanced input 1/4” combo jack: 10 kOhms
FET Limiter: Attack: approx. 100μS; Release: 10mS to 2Sec; Ratio: better than 20:1
ELOP Limiter: Attack: approx. 10mS for 6dB GR; Release: 2.5 Sec; Ratio: 10:1
Frequency Response: 5Hz to 60KHz
Maximum Output: +32dBm, +30dBm (into 1KΩ load)
THD+N: <.05% @ 1KHz
Dynamic Range: 115dB typical
Output Impedance: 200Ω
Power Consumption Slam! Analog:
0.480 Amps (480 milliamps = 480mA) @ 120V = 57.6 Watts
0.240 Amps (240 milliamps = 240mA) @ 240V = 57.6 Watts
Outboard Power Supply: factory set for 100V, 120V or 220-240VAC operation for original destination country's mains voltage.
Operating Mains Voltage: changeable with power transformer re-wiring via switch and fuse value change.
Mains Voltage Frequency: 50~ 60Hz
Size: 19” X 12” X 3.5” (occupies 2u)
Shipping Weight: 25 lbs.