It occurred to me tonight that we can probably trace this in Cyril Fagan's writings, since his monthly "Solunars" column in AAM began in 1953 So I pulled out my bound photocopies of the first several years of Fagan's work and did some digging.
As late as the March 1957 issue of AAM, when he gave Benito Mussolini's horoscope, it is clear that he is still using the Hypsomatic Vernal Point (Spica at 29°00'00" Virgo). The same is true with Hitler's chart in the April 19587 issue: All planet positions are about 6' off from where modern calculations would give them. The transits for Paul Clancy's death, shown in the May '57 issue, are quite badly calculated, but where they are not way off (e.g., Sun is 0°31' as if an AM/PM mix-up occurred, which would also explain the 5° Moon error), the difference is about 6'. Same with Mrs. Clancy's SSR in the July 1957 issue, the very one in which Garth Allen's final article on the subject appeared. (Of course, these were written months before the issues came out.)
Finally, in the October 1957 issue, a new copy of Mrs. Clancy's chart appears with Sun at 2°19'39" Aries. This is 01" from the best modern calculations using the SVP. By that time, Fagan is using the SVP, in an issue that hit the stands 5 months after the first Bradley article on the discoveries.
CONCLUSION: Fagan didn't know about the new discovery until approximately the time is was published. (He could have been holding it back to not steal Bradley's thunder on the discovery, but he likely wouldn't have done that for a long time, like a year or two.)
There is a brief acknowledgement by Fagan:
Chart 1 is our editress' birth horoscope. It will be noted that the longitude of all the planets and cusps of the houses have been increased by 0°06'05' to accord with Mr. Garth Allen's remarkable discovery of the synetic vernal point (S.V.P.).