* TH700R4: A 4-speed automatic overdrive transmission. This was very common.
* SM465: A 4-speed manual transmission.
* NV4500: A 5-speed manual transmission (less common in '89, more likely in later years of the C/K platform).
To know for sure which transmission is in a specific 1989 Chevy Silverado, you'd need to check the vehicle's identification number (VIN) or look at the transmission itself. The VIN can be decoded to reveal the original transmission code.