[Kevin Kuehn]

After looking at that circuit a while longer, those two diodes may be nothing more than a rf gain clamper. In other words they limit how far the if grids can swing positive. Apparently that contrast method didn't work very well because most manufactures soon abandoned it in favor of moving the contrast control to the video amp stage. Then AGC was used to control the if gain. Interestingly the same thing happened in the early days of radio when AVC came on the scene.