Training data attribution scores how training examples affect downstream model performance. For models released with many intermediate checkpoints, attribution can be computed along the saved training trajectory, but evaluating every checkpoint is often prohibitively expensive. We introduce PrAM (Probe-based Attribution Mass), a post-hoc offline checkpoint selector for training data attribution. PrAM runs a small attribution probe at every checkpoint, selects checkpoints with high absolute attribution mass, and runs full attribution only on that subset. Across Pythia-14M, 70M, and 160M, PrAM closely matches a full attribution-pool reference and outperforms heuristic rules based on recency, spacing, learning rate, and validation loss. Beyond rank correlation, PrAM preserves the domain-by-target attribution tables and ranked training examples produced by full saved-checkpoint attribution, enabling scalable post-hoc analyses of checkpointed model releases. We support this with application-based analyses scaling to 410M and 1B parameter models.