HO 5050 doesn't appear in the 1939 catalog of published HO charts. I don't think I have ever seen an image or description of HO 5050, other than Randy's letter, quoted earlier in this thread, which describes it as:
"Examining the Lexington search map, the fonts and locations of the islands on the Lex map match Murfin’s map. The Lex map was produced by the Hydrographic Office, and is called a No. 5050, Strategic Plotting Chart No. 17, covering a bit below -8*S to 16*N, and 175*E to 147*W. It too has a bit extended on the left hand side, but the chart is complete. This chart covers the Phoenix Islands and has their positions and names. The tick marks for longitude are located at -5*S and 10*N, and for latitude at 155 and 175* W. What is different between the Murfin map and the Lex Map is “Pacific Ocean” is not on the Lex Map spanning the two circular plots."
There may be multiple sheets, but I thought HO 5050 would "do nicely" for Fred Noonan on the basis that it extended south to 8 degrees. A chart to 16 degrees North would not cover the Hawaiian Islands, which are at 19 degrees North, so Noonan would need another chart for the Howland-Honolulu leg. Lae is 147o W, 7o S. Howland is 174oE, 1oN. However, the Hull Island is roughly 172o W, 4oS, and Niku is 171oW, 3oS, so the Phoenix Group would be off the map to the East.
Also note that if HO 5050 is dated 1924, it definitely would not have the advantage of any mid-1930s improvements in the position of Howland Island. So, it might be worth checking the position of Howland, either on the Lexington search map, or any 1920-1933 era unrevised nautical chart.
Finally, my guess is that HO 5050 represents an earlier generation of plotting charts, replaced by the VP- and VR- series later used in WW2, beginning in the 1930s, and using updated base maps. If Noonan obtained one or more unpublished plotting charts from the Navy (entirely possible but undocumented), it is anybody's guess whether he obtained the new ones or old ones.
adr