It was not until April 24, 1978 that Zodiac had directly addressed to Toschi in his (first) Herb Caen letter. Handwriting appears authentic to me and has Toschi been cleared of faking this particular letter (http://www.zodiacciphers.com/the-1978-letter.html
Assuming it to be authentic for a second, it raises the question why Z had addressed Toschi. IMO there might be only two reasons for that:
- Toschi had successfully investigated Z in his 'real life', thus Z nearly got busted and he coped with it by mentioning Toschi in his next letter (as another 'hint')"He will get tired, then leave me alone"
(Z implying that Toschi had not only once investigated him in his real life)
- Z had spoken in general about Toschi giving up his case, however he referred to 'Tales of the City' where Toschi was present as an investigator but Z felt that he'd be 'better'