This number is a prime.
This prime is 12*39^5+30*39^4+27*39^3+18*39^2+24*39+28 =C*39^5+U*39^4+R*39^3+I*39^2+O*39+S (A=10...Z=35). 39 is the smallest base in which a prime CURIOS occurs. Note that CURIO is divisible by 3 and thus there is no prime CURIO. [Rupinski]