jack screw bolt to be installed in the Manway cover flange

the jackscrew holes are Probably to remove the manway cover

...especially if the gasket is sticky.

In Pressure vessels, manways are covered by a thick dummy flange called manway cover. Dummy flange is put with the help of studs tightened in the several flange holes. When we want to take out the dummy flange after several months/years of operation it is difficult to remove because it sticks with the gasket. To make the job of removing easier it is normal practice to give provision for jack screws. It is nothing but few more threaded holes (other than flange holes which are not threaded) on dummy flange in the same pitch circle diameter. For removing manway cover, first all the studs in the flange holes are opened and then the jack screws are tightened to loosen the cover from the vessel side flange.