1981 Yamaha SRX440E Service and Repair Manual
Yamaha part number LIT-12618-00-38 is the service manual for the 1981 SRX440E. It was issued for that model on its own, with no other years or variants sharing the volume.
On a machine this old the manual is mostly consulted for figures that cannot be worked out any other way: engine clearances and wear limits, torque values in sequence, carburetor jetting and ignition timing settings, and the wiring diagram. The procedures around them are the dealer ones, written for a technician with tools rather than for a reader.
Beyond the engine, expect coverage of the clutch and drive arrangement, track and suspension adjustment, steering, brakes and the electrical system, along with the maintenance schedule Yamaha specified when the sled was new.
Order by the part number if you want to be certain, since the SRX designation appears on several quite different Yamaha machines.