They are propeller guards, and their purpose is to insure that the screws are not fouled. The model shows them correctly positioned.
Frank Knox carried  measure 33 when she was commissioned.
As to the other I don't know but will find out. Watch this space.
The following Gearing Class DD's carried Measure 21. Most of them that carried this measure were shipyard carrying out Navy instructions that were overcome by the events of the war ending. Some of these commissioned in 46 yet retained the wartime measure when the Navy accepted the ship into service.
Frank Knox 742, Southerland 743, Rowan 782, Gurke 783, McKean 784, Henderson 785, Richard B. Anderson 786. James E, Keyes 787, Hollister 788, Eversole 789, Chevalier 805,Higbee 806, Benner 807, Dennis J. Buckley 808, New 818, Holder 819, Johnston 821, Samuel B. Roberts 824, Power 839, Bausell 845, Ozbourn 846, Robert L. Wilson 847, Richard E. Kraus 849, Leonard F. Mason 852, Fred T. Berry 858, Norris 859, McCaffery 860, Harwood 861, Cone 866, Brownson 868, Arnold J. Isbell 869, Damato 871, Hawkins 873, Perkins 877, Vesole 878, Leary 879, Dyess 880, Bordelon 881, Furse 882, Newman K. Perry 883, Orleck 886, Brinkley Bass 887, Stickell 888, O'Hare 889, Meredith 890
Now back to Measure 22. I am afraid I cannot help you on the substitution of Navy Gray for Navy Blue in this measure per the 1945 instructions, Don't think anyone else can either. It seems that the Navy was running out of blue pigment in 1945. Instructions said to use up existing stocks of Navy Blue then switch over to Navy Gray. That is the short and dirty narration of it. I have never quite bought that story, because it seems that someone had enough in stock to paint all those DD's listed above in Navy Blue.
Anyway, the photos that are available are mostly in black and white, and it is near impossible to tell the difference between the Navy Blue and Navy Gray. If you were to ask me which one would be the safer bet for Measure 21, I would say go with the Blue