Used power amp in good full working order.
Supplied with power lead but no manuals
The odd mark to the case but on the top and sides.

From Rotels web site:-
Rotel’s new custom power amplifiers address all of the demands of the professional custom installer: easy rack-mount installation, failsafe design, and set it and forget it operation. They also come with the most important features that all music lovers want: great sound and an affordable price. This is all possible due to Rotel’s Balanced Design Concept, a design philosophy that emphasizes an exhaustive analysis of every part that goes into our circuits to determine not which one will do, but which one will sound best. There are three models to cover almost every custom application. The RKB-250 features, 2 x 50 watts RMS @ 8 ohms, the RKB-650 is rated 6 x 50 watts RMS @ 8 ohms, and the RKB-2100 output is 2 x 100 watts @ 8 ohms. These power amplifiers are extremely robust and are able to increase their output power into 4 ohms (20 Hz-20 kHz, 0.05% THD). Each RKB power amplifier includes front access gain controls for easy settings, a security cover for tamper-proof installations, an IEC detachable power cord, a 12-volt trigger input for remote system operation, and rack mount capability. With high output power, high thermal stability and short-circuit protection, the RKB Series is the ideal solution to any permanently installed sound system.

Model	RKB-650
Continuous Power Output into 8 ohms (20-20k Hz, < 0.03% THD)	6x50 watts/ch into 8 ohms, all channels driven
Continuous Power Output into 4 ohms (DIN 1 kHz,1.0% THD)	6 x 80 watts/ch into 4 ohms, all channels driven
Total Harmonic Distortion (20Hz-20kHz, 8 ohms)	Continuous rated power: < 0.03% One-half rated power: < 0.03% 1 watt power: < 0.03%
Intermodulation Distortion (60 Hz : 7 kHz, 4:1)	0.03% maximum
Frequency Response	15 Hz - 100 kHz (±1dB)
Damping Factor (20 - 20,000 Hz, 8 ohms)	>200
Input Impedance	20 k Ohms
Input Sensitivity	1.0 volt
Amplifier Gain	26 dB
Peak Current	> 25 A
Frequency Response (±1 dB)	10Hz-100kHz
S/N Ratio (IHF A)	115 dB
Crosstalk/Separation	> 70 dB
Speaker Impedance	4 ohms minimum
Auto Turn On Level (with all inputs)	3 mV input signal
Power Requirements:	USA: 120 Volts, 60 Hz Europe: 230 Volts, 50 Hz
Power Consumption:	450 Watts Idle: 42 Watts Standby: 2.6 Watts
Dimensions (W x H x D): not including rack adaptors	430 x 144 x 429 mm 16.9 x 5.7 x 16.9 in
Weight (net):	15.5 kg, 34.1 lb.
Panel Height:	3U, 132.6 mm/5.2 in