What you have is known as a surface plate ! These are commonly used for model making and fixture building ! The groves in it are to anchor fixturing tools , such as angles and risers ! You can buy the anchors at "Presision tool" or other tooling suppliers ! Most shops swithed to Granite surface plates because they were not afected by tempetures as much .. steel plates will flex , shrink , and grow !
Who ever buys it .. sand it down with some 220 wet/dry on a good flat block about 8" x 8" and some mineral spirits ... It will take all the surface rust and burs off .. then paste wax it ! "Johnsons furniture wax"
Good luck !