The biggest difference in function between a blade shutter and a slit shutter is the ability of a slit shutter to allow exactly the same exposure time for the entire light field collected at the front of the lens and practical use to allow faster shutter speeds.
Due to the fact that the shutters are opened longer in the middle than at the edges, the light that falls through the center of the lens falls slightly longer on the image plane than the light that comes from the edges of the lens. This was not such a big issue at the beginning of the photograph, and the emulsions were so low in sensitivity that the typical exposure times were in minutes rather than hundredths or thousandths of a second! In fact, the first "shutters" were lens caps or plugs that were manually removed and replaced on the front of the lens.
As cameras became more photosensitive and the desired exposure times became shorter and shorter, the limitations of the blade shutter became a more important problem. Nevertheless, today there are still new digital cameras that work with shutters. The designers agree and the market seems to agree that the compromises are worthwhile in some cases.
A slit shutter can be designed to start exposure on one side of the frame and terminate on the other side of the frame. In this way, all parts of the frame can receive light from all parts of the lens for the same time. The earliest single curtain shutters, as used in the speed graph, had a fixed slot that ran through the focal plane. By allowing the user to select different slot widths and spring loads for the mechanism that moved the slot across the focal plane, shutter speeds in the range of 1/10 second to 1/1000 second were possible with most different models of the Speed Graphic.
Why should the Speed Graphic have both a focal plane and a blade shutter? That does not have to be need a shutter. Tubular lenses without wing lock can be used with a Speed Graphic. The slot shutter is used for speedIn particular, shorter shutter speeds, so the name speed graphics, But it certainly was not the camera fast Regarding the shot-to-shot intervals and the operation of the FP shutter, manually resetting the FP curtain between shots took longer than operating a shutter in the lens. This may be one reason why many users prefer both options. The lens-sealed lenses offered by the lens manufacturers can be used on both the Speed Graphic, Crown Graphic and Century Graphic models. (The lack of a focal plane shutter made it possible to thin the Crown Graphic a bit, allowing it to use some wide-angle lenses faster than the Speed Graphic did.)
Although this is not exactly true for your particular model, here is a link to the instructions for a top speed graph of about 1925.