Can someone advise me on whether the AH Hirsch 20yr has bourbon from Michter's distillery...?
Yes, it is/was Michter's (Shaefferstown, PA) whiskey.
...I have read in Michael Jackson's-Whisky, The Definative World Guide...that the 16yr old is from this distillery and I read that it was bottled at BT, is it the same for the 20yr old?..
The 16- and 20yo Hirsch bottlings are from the same Michter's whiskey, distilled in 1974. The age difference is the difference in time between when they were removed from their barrels and tanked -- the 16yo in 1990, and the 20yo in 1994. There were also 190 cases of a 19yo bottling in 1993. A.H. Hirsch was bottled originally by Julian Van Winkle at his Old Commonwealth Distillery bottling line in Lawrenceburg, KY. Only the last bottling run -- the gold-foil capsule 16yos, in Sept. 2003 -- was performed at Buffalo Trace.
...Is there any current info on Michter's?..
It is defunct, and has lost its National Historic Register designation, and will almost certainly be dismantled. See here for some historical info:
Legacy of Bomberger/Michter's Distillery