Generalized repunit prime (base 249).
For base 249, I found R5, R1249, R2053, R3319, R8627.
For base 250, I found R2, R127, R1889.
For base 251, I found R7, R13, R17, R89, R227, R461, R3467.
For base 252, I found R541, R947.
For base 253, I found R19, R2659.
For base 254, I found R5, R19, R79, R283, R563, R883.

To be completed...