In this case, the illuminator has mistakenly marked the tun "MOK", but the first page of the text includes the arms and name of John Morton, so we can be confident of the identification. (It it were not for the lettering and other evidence, one might guess that this border was painted for someone called something like Eagleton, or Hawkton.) A second example of Morton's arms and rebus is here.
A web search finds lots of examples in other media:
|Shell + tun = Shelton|
|Mill + tun = Milton|
|Beacon + tun = Beckinton|
|Bolt + tun = Bolton|
|Pastoral staff over a tun = Abbot Overton's rebus|
|Bar + tun = Barton|