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Side Valve 1937 U Grainy photo but we post it here for your reference.

Service Shop Dope

No. 252 November 1, 1946

Tires Wheels Speed

When a racing motorcycle or car Is being groomed for an event on road or track, just as close attention is given to perfect condition of wheels and tires as to engine tuning for maximum performance. Wheel bearings are checked, wheels and tires are checked for out-of-true sideways, eccentricity, out-of-round, and out-of-balance. Careful attention is given to anything found not just right. If a tire tread is found worn irregular to an appreciable degree, tires are transposed or a new tire installed. Inflation pressure is carefully adjusted to the poundage known to be right for weight of vehicle and nature of event.

The model number is in effect the motor number of a bike. The motor number is stamped on the left crankcase. The numerals of the motor number preceding the letters indicate the year. The letters indicate the size and type of the motor.

For example:

41EL2037 would be a 1941 model with a 61 OHV motor.
38U1863 would be a 1938 model with a 74 SV motor.

BT: Big Twin
Flathead: A type of engine with Side Valves.
Knucklehead: A type of engine with Over Head Valves.
OHV: Over Head Valve
SV: Side Valve