Looking for MS # R-50: The German Situation in Italy, 11 May–4 June 44

During research pertaining to Italy in 1944, I came across intriguing information from the German perspective during the Battle of Anzio cited in Ernest F. Fisher, Jr.'s book "Cassino to the Alps," one of the official Army histories (aka Green Books). It was cited as MS # R-50 (Bailey), CMH or Bailey, Britt, “The German Situation in Italy, 11 May–4 June 44.” July 1951. OCMH, R-50. In the sources section, he mentions studies "produced by staff historians with the Foreign Military Studies Section of the Center of Military History. Grouped in a so-called R-Series, they total 165[.]" Some of the other CMH foreign series (A, B, C, D, P, and T) are available on Fold3, but I have been unable to find any archive with the R-series. It doesn't look like the report is in the US Army Heritage & Education Center collection either.

I checked with the Center For Military History (CMH). They do not have any of the R-series available. The CMH referred me to NARA, explaining that "background files of CMH publications are ultimately accessed by the National Archives."  I was hoping to find out if there was a copy of R-50 in Background Files to the Volume "Cassino to the Alps", ca. 1950 - ca. 1977 Background Files to the Volume "Cassino to the Alps" (archives.gov) or elsewhere at NARA.