In other words, I'm confused because I don't understand which sets you're comparing and/or the relationship you're trying to describe. As an example, a 1960 horn could have an overhaul in 1965 and then not be touched until 2010. You may be able to determine that the horn is at least 45 years old if and only if an overhaul technique was discontinued in 1965.
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