The Company recorded 660 x 74 = $48,840 in deferred revenue. This is a liability account that means they still owe the service or good which they have been paid for.
Every month, the company records an adjusting entry, recognizing one twelfth of the 48,840, 4,070, because they have earned another month of that deferred revenue by providing the magazine.
The journal entry on December 31, 2018, will be
Deferred Revenue 4,070
Revenue (4,070)
If this is the first time they've made the entry, then they will recognize earned revenue for Sep, Oct, Nov and Dec,
Deferred Revenue 16,280
Revenue (16,280)