I just noticed another maker in the picture.
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I think you are probably right it is the same area. In the old picture, you can see a road behind the marker you point out. However, that road may have disappeared over time. It would be interesting if you can walk behind the trees and brush when you visit Vicksburg in July, to see if there is topography similar to what is shown in the old picture. It would appear that the earthwork with the cannon and plaque has been leveled at some point. Although, it is possible that it is obscured by the present day trees and brush.
If you google "43rd Tennessee" it will take you to a website (made by descendants of the 43rd Tennessee, who were helping out the Georgia regiments at the Salient Work), then click on "Vicksburg pictures no.1" and scroll through the pictures, and you will see some pictures from when the descendants of these Tennesseans visited Vicksburg in 2008 I think and tried to find out where the Salient Work was located. Also, there are pictures on CivilWarAlbum, click on Vicksburg campaign, then click on Hall's Ferry road, there are neat pictures. One says "Confederate earthworks" but the picture doesn't show very much, only vegetation really. There is a medium size plaque for Cummings Brigade which plaque was where the Salient Work was located (supposedly).
But you have done a great service for me and those of us interested in these earthworks. If you get any good pictures from your visit in July, please post them on here.
Parker Hills said that he will typically take a tour group and park on John Allen Street, and walk north, and there are remnants of trenches in the underbrush. So, if you are so inclined you might do likewise.
Last year I was able to visit Vicksburg, and I climbed down in the ravine across the Baldwin Ferry Road from the 2nd Texas Lunette, and found a marker for the start of A.J. Smith's approach to the Lunette. It was rough walking but I got some pictures.
I think that there is a very strong possibility that your identification of the Confederate trench on Iowa Avenue is correct. If you get a chance to walk it, you can probably confirm that. Vicksburger