Letters should be sent to those, but it's not guaranteed. It's best to get a stamp at 23 months of waiting (they won't give appointment earlier). It's a risk though, because often when giving stamp, USCIS takes GC. And if LPR receives 48 month extension letter later, it would be useless. Stamp is only good for 1 year, whereas letter may be giving 2+ years of extension, depending when LPR receives it.