That would be the odss of being dealt quads by the flop. If you already have a pocket pair then the odds of flopping quads would be the lower figure of ~410-1 quote above.
True... But the odds of recieving a pocket pair (17-1 or 5.55556%) and then improving to quads (407-1 or 0.2451%)
=0.0136% or approximately 7343 - 1