D W Onan & Sons

To break down the Onan part number, the following may be of help:

Example:
2.5MAJ-51CERV / 1M This is NOT a practical number but serves to illustrate the details
2.5	Output rating of the generator in VA or Watts, depending on the age of the unit and if it is DC or AC output.
MAJ	Engine Type	AJ 2.75" X 2.5" 14.9cu in 1-Cyl Side-Valve 6.25 CR AK 2.5" X 2.5" 12.2cu in 1-Cyl Side-Valve 5.5 CR BH - 2.5" X 2.25" 22.10cu in 2-Cyl Side-Valve 6.1 CR CCK - 3.25" X 3" 50cu in 2-Cyl Side-Valve 5.5 CR CCKB - 3.25" X 3" 50cu in 2-Cyl Side-Valve NA CR CW JB JC LK 3.25" X 3" 25cu in 1-Cyl Side-Valve 5.5 CR MAJ - 2.75" X 2.5" 14.9cu in 1-Cyl Side-Valve 6.25 CR MCCK - 3.25" X 3" 50cu in 2-Cyl Side-Valve 7 CR MDJA - 3.25" X 3.625" 30cu in 1 Cyl OHV Diesel 19 CR MDJB - 3.25" X 3.625" 60cu in 2 Cyl OHV Diesel 19 CR MDJC - 3.25" X 3.625" 120cu in 4 Cyl OHV Diesel 19 CR NH - 3.562" X 3" 60cu in 2-Cyl Side-Valve 7 CR
5	Output Frequency and engine speed	5 = 50Hz 1500rpm or 3000rpm no number = 60Hz 1800rpm or 3600rpm)
1	Output Voltage and connection	1 = 120V 60Hz 2-Wire 2 = 240V 60Hz 2-wire 3 = 120/240V 60Hz 3-wire 4 = 120/208V 3-Phase 4-Wire 5 = 240V 3-Phase 3-Wire 5D = 120V/240V 3=Phase 4-Wire 115 = Magnet service 115V DC Output 150 = Magnet service 250V DC Output 206 = Battery Charging Set 6V DC Output 212 = Battery Charging Set 12V DC Output 224 = Battery Charging Set 24V DC Output 232 = Battery Charging / Lighting Set 32V DC Output
C	Output Connection - Special Four-Wire reconnectable Series/Parallel output
E	Electric Start - (Where the specification would otherwise be Hand Starting)
R	Generator Style	M = Fixed Stationary (Can be Electric or Manual Start) P = Portable (Always Manual Start) R = Remote (Always Electric Start) A = Automatic (Always Electric Start)
V	Vacu-Flo Cooling (In place of normal centrifugal fan cooling)
/1M	Factory Codes for build options etc.

I will expand on this later when we have more time, and fill in more of the output voltage and connection types.