Two models from the same lab released within two months of each other are much more alike (0.58 cosine similarity) than models from different labs released at the same time (0.20). Eight or more months apart, a lab's models are no more alike than models from different labs (−0.24 for both).
Subtract what's in fashion, and Opus 5.5 lands almost opposite Astra (−0.45). What's left is each lab's own handwriting.