I'm looking for some help in understanding the second line below as I'm not sure I've understood it correctly. Is it saying that the jeweller mentioned in the first line was commissioned to perform jewel appraisal for the last several decades? I'm a bit confused here as to how 年分 works.
「例の偽ダイヤを集めてた宝石商は？」
「はい。やはり過去数十年分のジュエル鑑定を市から委託されていました」
It's "several decades' worth of appraisal" - the implication is that there is a certain amount of work to be done every decade (of appraising jewels), and the amount does not change much decade by decade, so you can count them by decades.
The frequency is unclear from the passage alone, by the way. It's possible that they did the appraisal only once in those decades in one big batch (perhaps unlikely), once every year, once every month, or more frequently than that.
A similar and more common example: If you get garbage collection every week and the collector skips one time, you will have accumulated two weeks' worth of garbage, or 2週間分のごみが溜まっています.
年分 means "years" or "period of time" in this context. So 過去数十年分 would translate to "the past several decades" or "the past several years."
"Yes. As it turns out, they had been commissioned by the city to appraise jewels for the past several decades."
