I've shot a handful of those firecracker red films. It's definitely a Chinese 100 speed film. Shooting a 100 speed film with red filter that typically a has a 3 stop reduction of transmitted light would give you an effective ei that's quite low (100-50-25-12). I'm surprised you found your negatives to be well exposed after processing with a two stop under exposure with old film.
I have two rolls left of these, does it look similar? The newer films are in a white with silver packaging. These are super curly after processing.
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