Green’s Dictionary of Slang

(US gay) a popular variety of post-gay liberation stereotype, often posing as a lumberjack type with checked flannel shirt, Levis, heavy boots etc.

1978		E. Guthmann in S.F. Sentinel 2 June 11: [the moment when] we all turn into Castro Clones […] buying the right levis and tee shirts, wearing the right bronzer, attending the right disco, sporting the right haircut.
1978		H. Caen in Ventura Co. Star (CA) 13 Aug. 11/2: Ifd you’d care to catch the Beautiful Boys in their headquarters or tailquarters, cruise out to Castro around 18th. They all look alike, neatly lean in their jeans, hair and moustaches short. Eward Guthmann is to be forgiven for calling them The Castro Clones.
1980		E. White States of Desire 45: [Y]oung men with clipped mustaches and hair, tight-fitting Levis, flannel shirts, workboots [...] the celebrated look of the so-called Castro Street Clone [Simes:DLSS].
1994–2003		(ref. to 1970s) KQED ‘The Castro’ homepage on KQED.org 🌐 The ‘Castro Clone’ look became a distinguishing feature of gay liberation in the 1970s, and signified a departure from previous stereotypes of gay dress and appearance.
1995–2003		S. Sheppard ‘Sex Talk’ at Gay.com 🌐 Then came the ‘Castro clone’ look, now a tired cliche, but then semi-revolutionary. Gone in the first flush of sexual liberation was the old butch-femme divide, replaced by flannel shirts and Levi’s 501s worn by tops and bottoms alike. It was an assertively butch look that said, ‘Real men do like to get plowed.’.
1998		R. Scott Rebecca’s Dict. of Queer Sl. 🌐 Castro clone — a gay man of the type who lives in or frequents The Castro. They all dress very similarly, tending towards very masculine attire and pumped-up, buffed good looks. The style is, in many ways, set by Tom of Finland.