W 601 (NGC 6611 601) is one of the handful of known magnetic Herbig Ae/Be stars. We report the analysis of a large dataset of high-resolution spectropolarimetry. The star is a previously unreported spectroscopic binary, consisting of 2 B2 stars with a mass ratio of 1.8, masses of 12 M$_odot$ and 6.2 $M_odot$, in an eccentric 110-day orbit. The magnetic field belongs to the secondary, W 601 B. The H$alpha$ emission is consistent with an origin in W 601 Bs centrifugal magnetosphere; the star is therefore not a classical Herbig Be star in the sense that its emission is not formed in an accretion disk. However, the low value of $log{g} = 3.8$ determined via spectroscopic analysis, and the stars membership in the young NGC 6611 cluster, are most consistent with it being on the pre-main sequence. The rotational period inferred from the variability of the H$alpha$ line and the longitudinal magnetic field $langle B_z rangle$ is 1.13 d. Modelling of Stokes $V$ and $langle B_z rangle$ indicates a surface dipolar magnetic field $B_{rm d}$ between 6 and $11$ kG. With its strong emission, rapid rotation, and strong surface magnetic field, W 601 B is likely a precursor to H$alpha$-bright magnetic B-type stars such as $sigma$ Ori E. By contrast, the primary is an apparently non-magnetic ($B_{rm d} < 300$ G) pre-main sequence early B-type star. In accordance with expectations from magnetic braking, the non-magnetic primary is apparently more rapidly rotating than the magnetic star.
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