If done in PSG, then I'm pretty sure it wasn't done with a style that's usable in PSG. There might be a PSP style for this, but if I understand what you want, a style isn't needed. I'm assuming that by
...where 9 or so frames come into view one at a time a...
you mean 9 images appear to be added to a slide one at a time.
If that's it, then by your description, this is what I call a 'build'. In the PSG, the way to do this is to create a series of slides, in this case probably 10 slides. I find it easiest to start at the end--with the one that has all 9 images on it. Get that the way you want it then copy it twice so there's 3 slides with 9 images on them. The last one isn't going to have anything happen at all, that's there so everyone sees the full group. The one before that is where image #9 is added. How it gets added is controlled by any motion you give that image layer and the transition at the start of the slide. Simplest approach is just a fade-in transition (but most transitions work). The trick is to set all the layers (the other images), except this last one, so they are 'above' the transition, they're not affected by it, but #9 is 'below' so the fade-in (for example) happens for that one.
Now back up to the one before that which is where #8 enters. #9 doesn't belong there so you can delete it (or turn that layer off) and now reset #8 to be 'below' transition and decide what (and how long) the transition into this slide will be which controls how #8 enters.
From there it's just working back through more copies deleting layers and setting transitions. There's no reason you can't do it from front to back but I find it much easier to set up all the positions on the final group first and then work back deleting the ones I don't want so I build up the final image.
This is actually a lot harder to describe in words than it is to actually do--once you've worked through it once or twice especially.
Hope that helps,
Dick