SKS is based off the PTRS design. Maybe the furniture is off the SVT-40 design.
And what is the PTRS designed from? Considering one of Simonov's direct supervisors before he created his designs was Tokarev do you think there could possibly be some influence?

Siminov first created the AVS-36 before the PTRS. Work on the AVS-36 began in 1930, the same time as Tokarev began working on his semi auto gas system. Some differences between the designs but look at the pictures and decide:
AVS-36
The AVS-36 was a gas-operated rifle with a short piston stroke and
vertical sliding locking block
Tokarev SVT38 (predecessor to the SVT40)
The AVS-36 came before the SVT38 however the design times were about the same, with Siminov's design winning the first competition and Tokarev winning the second.
The SVT38 was a gas-operated weapon with a
short-stroke, spring-loaded piston above the barrel and a tilting bolt. According to Wikipedia "There is some dispute about who exactly first developed this operating principle, as the SVT's mechanism closely resembles Dieudonne Saive's contemporary designs; Saive eventually designed the FN FAL, which employs the same operating principle as the SVT".
Of particular note was that "The
SKS is a gas-operated weapon that has a spring-loaded operating rod and a gas piston rod that work the action via gas pressure pushing against them. Also, it has a "tilting bolt" action locking system (Wikipedia). Note that this is the same system as the SVT38 and SVT40 unlike the very similar previous design by Siminov the AVS-36 which didn't use a tilting bolt action.
Now ad in the fact that the SVT40 was pretty much the only successful semi auto rifle on the Eastern front. The Germans tried to copy it with their semi auto but didn't meet with as much success.
When you look at the STG44 system, you might want to think where some of that inspiration came from. The German's semi auto system was copied from the SVT40 when their designs failed. I don't have a STG44 but my CZ858 which more closely resembles the STG44 than the AK has an eerie resemblance to my SVT40 rifles.
I personally believe the SVT40 was the great grandfather of the current assault rifles. With the German STG44 (and prototypes) being the father of the modern assault rifle.