Indeed, only the ER4 series come with the "certificate of performance". Back before Etymotic were acquired by Lucid and the ER4 series IEMs were still manufactured in the US, they released the ER3 series as kind of a cheaper priced, slightly differently spec'd (lower impedance and therefore higher sensitivity, which also results in slight tonal differences compared to the ER4 equivalents) alternative to the ER4 and successor to the HF5 (manufactured in China vs the US (which, so I was told back then, was the main reason why they could be offered quite a bit cheaper), less strict quality control with more allowed production tolerance (but, so I was told, and so my personal measurements of the ER3SE and ER3XR show as well, in reality this shouldn't make any difference and most, if not all, ER3 series IEMs should perform within ER4 series production tolerance specifications, but unlike the ER4 series, they could not guarantee this (-> "certificate of performance")), no individual unit quality control by hand (hence no "certificate of performance"), less accessories, different cable).
Now with all of the talk about this "certificate of performance", looking at the ones I got with the ER4XR and ER4SR (late 2016), I am curious and wonder who "K.M.", who performed and signed both, is.
@EtyDave