I own a Standard Range 2019 model 3
What it is is a Standard Range Plus. But Tesla eliminated a few speakers (which are in the car and all I have to do is buy a $50 harness and hook them up). Same battery pack as the Plus but Tesla, by software, shortened the mileage by 40 or so miles.
I special ordered the SR because I didn't need the mileage and I loved buying it new for $35,000 less $2500 rebate from Calif and $1000 check from So Calif Edison making the cost of my car $31,500 which was cheaper than a Toyota Prius I was considering.
Originally, I think Tesla was going to make a SR with different seat coverings, roof, etc., etc. but decided it was too expensive to do it that way, So, they took the RS+ and took away a few items, sold it as the SR for awhile to full fill a promise made by Elon.