They are the firstfruits of Christ, not an ethnic people, but a spiritual people. Which is why in Rev 14 they are depicted as being in heaven spiritually alive. Yes, these Old Covenant faith saints were Jewish, but more importantly than that, they were of faith, believing Christ could come to redeem them from death. And Christ did come so they are the firstfruits of Christ to ascend with Him spiritually to heaven. No different than what Paul tells us in 1Cor 15:20. Since Christ is risen from the dead He became the firstfruits of physically dead saints.
1 Corinthians 15:20 (KJV) But now is Christ risen from the dead, and become the firstfruits of them that slept.
Before Christ ascended to heaven after His resurrection, He first descended into the place of physically dead saints. Then Christ took them spiritually alive with Him to heaven when He ascended to the Father there.
Ephesians 4:8-10 (KJV) Wherefore he saith, When he ascended up on high, he led captivity captive, and gave gifts unto men. (Now that he ascended, what is it but that he also descended first into the lower parts of the earth? He that descended is the same also that ascended up far above all heavens, that he might fill all things).
"They ARE messianic ethnic Jews." Exactly, I wholeheartedly agree! And they are indeed the Church of Old, or the Church in the wilderness.
Acts 7:37-38 (KJV) This is that Moses, which said unto the children of Israel, A prophet shall the Lord your God raise up unto you of your brethren, like unto me; him shall ye hear. This is he, that was in the church in the wilderness with the angel which spake to him in the mount Sina, and with our fathers: who received the lively oracles to give unto us: