The Tale of 2 Pamela Voorhees Sweaters- Friday the 13th (1980)

I wish that I could take credit for this observation, but all kudos here go to u/camper of the Friday the 13th subreddit, a.k.a littlegrotesquerie of the Friday the 13th Fanatics forum. The user started a project in which she actually knitted her very own Pamela Voorhees Sweater.

Hand-knitted sweater by littlegroteserie

As you can see above, littlegrotesquerie did a phenomenal job at knitting this sweater. They were able to do this by freeze-framing the film and counting the individual stitches, determining yarn thickness, and the sweater's construction. While doing this the user also noticed that Betsy Palmer wears two different versions of the iconic sweater! I am not surprised that it took as long as it did for this to go noticed, as the differences are very slight. It appears that the sweaters are the same brand and that the differences likely come from a change in pattern due to sizing, as sweater with additional details fits Betsy a bit larger.

​Difference 1: From center outward, the "primary" sweater has a tight 4 stitch-wide band, followed by a cable on each side, followed by a tight 2 stitch-wide band, then the "diamond" stitched pattern, another tight 2 stitch-wide band, and lastly a cable that runs from the waist band before transitioning into a plain band around the armpit area. On the "secondary" sweater the 2 stitch-wide band that lies third to center has a large space in between.

1st difference between the primary and secondary sweater

​Difference 2: After the diamond pattern, the "primary" sweater has a tight 2 stitch wide band running from the waistband to the shoulder seam, followed by a cable running from the waistband until right around the armpit, where it transitions into a plain band that runs to the shoulder seam. On the "secondary" sweater, this cable stays one rather than transitioning to a plain band.

2nd difference between the primary and secondary sweater

As of now, these appear to be the only differences between the two sweaters. Both are complete plain on the back and have a single cable running down the arm. When it comes to determining which is the "primary vs "secondary" sweater I reviewed the film from Betsy's first appearance on screen to the very last scene of Savini wearing the severed head rig and grasping at the air. The "primary" sweater was labeled as such because it appears in some of the most pivotal scenes including the motive rant and the beheading. The sweaters alternate almost every other scene, as follows:
1. Pam's first appearance up to the motive rant- Secondary Sweater
2. Motive rant to the end of the first fight with Alice- Primary Sweater
3. From Pam finding Alice, through the second fight, to Pam breaking through the door- Secondary Sweater
4. Pam walking through door to Alice rolling her over on the floor- Primary Sweater
5. Beach attack and fight- Secondary Sweater
6. Savini FX Beheading scene- Primary Sweater