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Must have gotten better at blending them in (or just the usual blindness). Spent longer looking for the last difference than the other five combined by a longshot. No luck.
Found 6 on Pair 387 before switching over. Stalled at 3-4 on Pair 388 for a while. Swapped back and forth. Checked for shadow lines, but not missing green lines. (Looked at the green lines to be connected, not missing.) Added that bit of cactus well. Eventually found the Seviper on 387. (Suspected something after leaving the entire bottom half of the image alone.)