I can get a good deal on a Ecrin Roc but it is 15 y/o.
Would you use a 15-year-old helmet with an unknown history?  :-\
I found this post but it is inconclusive https://ukcaving.com/board/index.php?topic=18218.0

Personally I probably would if you like the fit of an ecrin roc. My caving helmet is almost entirely to stop me bashing my head on the ceiling rather than deal with big forces. I've also drilled holes in it to mount lights on it, which would probably be punched into the top of my head if a massive rock fell on me, or I took a huge fall onto my head, or something.

Nope nope nope nope. Save up and buy a new helmet. There is only 10 years life on these from date of manufacture anyway but with no history, it's a ridiculous gamble to take.