So, I have heard that 1TRZ was only concerned with the furniture is that correct?
My M59 has a matching serialed stock, there are two huge x's across the butt over the serial vertically...but I chalked those up to trench 'art'? It is stamped 1TRZ on the stock, but not the handguard as I have seen several times before.
The stock seems refinished vs. the handguard. The handguard is kind of discolored by filth...the stock seems as if it likely was too...but cleaned and then lightly coated with what appears to be shellac. But, perhaps the wood just seems to have a sheen on it from handling?
There was enough internal cosmoline left that I don't think this had been fired since importation...and it seems competely off base for the PO to have refinished the stock, skipped the handguard...and never bothered to clean the gun? Is this along the lines of something that sounds 1TRZish?
Unfortunately, I have handled precisely one M59...this one. I have to live vicariously through the internet...
It appears matching, Yugo style anyway...the carrier penciled on the side as is the trigger frame. The gas tube, likely isn't original...there is BX something penciled on the tube itself under the handguard and a 47 penciled under the front end on the bottom flat...47 has nothing to do with my numbers. The handguard ferrule has at some point been removed...yet does not appear to be shellacked and is a considerable mismatch in appearance. This lends me to think the PO didn't refinish it...as they likely would have. (perhaps at a light cleaning the PO accidentally switched my gas tube with another before consigning)
Anyway great thread GM...this is my go to spot for info on this model. 
Well.... It's apples to oranges.. but I have several variants on Yugoslavian Mausers.. I have one particular M48a that has a birch stock and elm handguard, and the dark wood stock/elm hand guard M59. I'm going to guess the stock is Birch, it's not elm and sure isn't teak. So would say any combination of wood is possible.
I've seen both scrubbed German Mausers and Russian M44 Mosins also sporting the 1.TRZ. In regards to wood... one has to remember they are dealing with a poor Communist nation who told Russia to basically take a hike. Dirty nasty wood would probably be acceptable for reuse, broken wood wouldn't, think of it as going green to save green. Given the vast array and type of weapons they refurbed, manufactured and used is kind of amazing, Looking at all of the Communist nation's, that little country cranked out massive piles of arms.
Yugoslavia also clung to the 8mm Mauser round pretty heavy vs. switching everything over to the Soviet 7.62x54r...even though they did have those caliber weapons and produce ammo, the 8mm was their go to.
Yugoslavia was very proud of their storage grease, a M48bo I bought, the whole barrel was filled... Not just a coating inside, it was like they took a grease gun and pumped the whole barrel full. I ended up with an impressive grease glob shoving that turd out.
To answer your very first question...
6) Some stocks can be found with 1.TRZ stamped on the right side of the butt stock. Which stands for Technical overhauling institute #1, TRZ is Tehnicki Remontni Zavod. It is located in Cacak Serbia, and was functional as early as 1944 based on weapons that predate the SKS found with said stamp, and was believed to be in operation as late as 2005. They are believed to be only dealing with wood and/or stock work.
There is also a Triangle BP124 stamp...Vojna Radionica 124 (Military Workshop 124) I have never seen this stamp on a SKS..... Mosins, Mausers and even some PPSh-41s and a few other pre 1950 weapons seem to be the ones with this mark. There is also a run of Radionica Yugo Mausers, they are fairly uncommon when comapired to the Preduzece 44 made weapons.