Those are the updated crystals. Every K2 after SN 2560 has them. The 4.9135-S is the thing to look for. 73, Don W3FPR On 1/6/2021 8:51 AM, Jim via Elecraft wrote: > K2 serial 397* with KSB2 how do I know if I have matched crystals? > > Reading the current crystal markings as follows: > ECS V > 4.9135-S > KOREA J3