L61R, AIII & AV are all the same thread pitch. 691 & 75 & I believe 85 are all the same pitch. The TRG-S is metric for sure because I had a butcher of a gunsmith friction fit a standard thread for me & this was the result.
I believe the L61R, AIII & AV are indeed 1x16 TPI. Someone may chime in & confirm this.
Amanda. May 11/1984 - Mar 21/2011. Smile down & watch over me & the kids.