  In an effective operator approach, the full set of leading contributions to
anomalous top couplings comprises various new trilinear as well as higher
interaction vertices, some of which are related to one another by gauge
symmetry or equations of motion. In order to study trilinear top couplings to
SM gauge bosons such as ttA, ttZ, tbW and ttg, the operator set can be
restricted accordingly. However, the complete basis cannot be mapped onto an
on-shell parametrisation of the trilinear vertices alone. Four-fermion contact
terms qqtt and udtb must be included if the relation to the operator basis is
to be retained. In this paper, we point out how these interactions contribute
to the single top search channels for anomalous trilinear tbW couplings at the
LHC and Tevatron, thus affecting the corresponding bounds. All results are
based on full leading-order partonic matrix elements, thus automatically
accounting for off-shell and interference effects as well as irreducible
backgrounds. A discussion of the quantitative effects of going from on-shell
tops to full matrix elements including acceptance cuts is also provided.
