This number is a prime.
The Old Believers in Russia believed that the end of the world would occur in 1669. [Robinson]
1669^2 = 2785561, and 278 * (5/5) * 6 + 1 = 1669. [Meller]
The smallest prime that appears in the same position of its own value when the Roman numerals (from 1 to 3999) are placed in lexicographic order. The other primes with this property are 3623 and 3631. [Meller]