When Paul talks about one body, his emphasis is not on an ecclesiastical body here on earth but on one Spirit, one hope, one faith, one baptism, one God. Rather than point to Peter as proof of the unity of Christendom, he points to the Spirit and the things pertaining to it for the same. The ecclesiastical body only matters insofar as the spirit and all those things are with it, and we can argue about whether it is in communion with Rome we find those things, but I don't believe this verse proves anything in that regard either way.
Ephesians 4:3-5New American Standard Bible (NASB) being diligent to preserve the unity of the Spirit in the bond of peace. 4 There is one body and one Spirit, just as also you were called in one hope of your calling; 5 one Lord, one faith, one baptism To me, the NASB (my preferred translation) indicates something like what Potter is saying, if I understand him correctly. It's focusing on the unity of the Spirit in the bond of peace. Well, the Spirit poured out and breathed life into the body of Christ - the church. We are one body of Christ, united through the Holy Spirit, and that is more important than denominations, ecclesiastical bodies, churches, etc. I infer that from what he could have possibly meant then - it was a united church, but separated by cities, distance, etc. Regardless of cultural differences, regional differences, and so on, it was one Spirit.
"3 Careful to keep the unity of the Spirit in the bond of peace. 4 One body and one Spirit ; as you are called in one hope of your calling. 5 One Lord, one faith, one baptism." Douay-Rheims Version