As so often with biographies I guess it could well be that Klaus - besides possibly misremembering things - wrapped up events a bit. He might have arrived in London in late 1963 and visited a recording session and he might as well have witnessed that early 64 recording session of "If I fell" but simply combined the events for atmoshphere...
I agree...
So, there are -at least- two possible scenarios:
1) Voormann goes to London by Autumn 1963, together with his Ramírez, Sutcliffe's Höfner 500/1, his new tape recorder and a few belongings. Bill Corbert pick him up at the station and they go to Abbey Road Studios, as Harrison had advised in his letter to Voormann. But they aren't -obviously- rehearsing IIF. Bill Corbert takes Voormann to R&G's Green Street flat and, when they return back home, Harrison sees The Ramírez for the first time (=naturally enough, Voormann had written in a previous letter about his purchasing of the guitar in Madrid, after leaving Tenerife...) On the next morning, Harrison takes the guitar with him to the studio, and the guitar is used there for the first time in an unknown song...
2) Voormann goes to London by end february 1964, together with his Ramírez, Sutcliffe's Höfner 500/1, his new tape recorder and a few belongings. Bill Corbert pick him up at the station and they go to Abbey Road Studios, as Harrison had advised in his letter to Voormann. There, they are rehearsing IIF. Bill Corbert takes Voormann to R&G's Green Street flat and, when they return back home, Harrison sees The Ramírez for the first time (=naturally enough, Voormann had written in a previous letter about his purchasing of the guitar in Madrid, after leaving Tenerife...) On the next morning, Harrison takes the guitar with him to the studio, and the guitar is used there for the first time in AILH...
I know it's pure speculation, and IIF plus AILH were recorded the same day...
Best!!
Xosé