One (free) example: a clarinet with a technical detail, first presented 1980, could be an older model, modified during an overhaul in the 80s. But a bore pattern of the early 20s with a model assumed to 1980 would be uncommon (if we know about a pattern change in the mid 50s). Therefore we can't estimate 100% sure dating in both directions.
Your opinion regarding the amount relations for low cost clarinets I would accomplish.
kindly
Roman