St Edmunds Hall
St. Edmunds Hall is recorded as the patron of St. Mary's Belchamp Walter in 1583. This is after the Reformation (1509-1559) and before the Interegnum (1649-1660)
St. Edmunds Hall is an Oxford college and their website gives no indication of why they would be listed as a patron of the Church in Belchamp Walter. The date of 1583 was 9 years after the chantry chapel was destroyed and the reasons for Queen Elizabeth's desire to remove "popish" elements from churches is not known.
Newcourt's Repertorium
Newcourt is probably the source of information on the clergy and patrons of St. Mary's church. The entry for Belchamp Walter confirms an Oxford patron.
The last column in Newcourt's table is not really clear and it looks like E.i. Oxon.
The vicar in 1583 is listed as Tho Corbett by Newcourt. This is also reflected in the Church Guide.
Elizabeth I
The patronage of St. Mary's definitely has something to do with Elizabeth I. This would be common for all what then became "Churches of England" as opposed to Catholic Churches.