The Top 20 chart dates from the week 11-17 September 1968 according to the information at https://www.officialcharts.com/charts/singles-chart/19680911/7501/
Also, in an interview Page gave to Chris Welch for Melody Maker , the article being dated 12 October 1968 , he says that LZ were due to go to the US in early November of '68 so perhaps the first US tour dates weren't firmed up at the time.