Releasing the top lever, here is how Parker Brothers describes it:
https://parkerguns.org/forums/attach...5&d=1623103172
https://parkerguns.org/forums/attach...6&d=1623103172
See the next to last paragraph of the upper hangtag (the brown one, not the yellow one).
Your description suggests to me that you found the right spot. Labeled “TRIP” on the pictures of the hangtag.
And if the firing pins are not visible, that suggests the hammers are cocked back. “Hammerless” (so called) Parker Brothers guns do not have a floating firing pin. The pin is on the hammer.
# 2 on this diagram.
https://parkerguns.org/images/tech_2.jpg
# 1 & # 2 on this picture.
https://parkerguns.org/images/parts.jpg
Hard to imagine but not impossible for pins to have been filed down.