bar instability. The growth of most bars in this scenario is gradual, with bars only starting to become discernable after ∼ 2 Gyr. Additionally, the central hole gradually vanishes in the bar unstable models as stars stream into the centre during the process of bar formation. By contrast, the models for scenario KL are far more susceptible to global instabilities than for scenario K, as the growth of the bar mode is rapid and nearly instantaneous. We therefore rule out models from scenario KL as being unable to properly model the galaxy; this conclusion is a setback for the hypothesis that the Type II SB profile is due to dust extinction. We next test fits to the gas and stellar rotation curves via a third MCMC chain corre- sponding to scenario KG. We fit the H β /H I rotation curves by assuming that they trace the circular velocity and ignoring the stellar rotation data. The dispersion and surface bright- ness are fit as in scenario K. The stars will rotate more slowly because of asymmetric drift (Eq. 3), an effect that is incorporated into the GalactICS disc. Examining the rotation curve fits in Fig. 5, it is apparent that scenario K cannot re- produce the H β /H I data and scenario KG cannot fit the stellar rotation data. Thus, it is likely that scenario K represents a more realistic model for NGC 6503. Note that both runs match the H I data beyond ∼ 3 kpc, since there is no asymmetric drift at those radii. The other fits, shown by the surface brightness in Fig. 6 and the LOS dispersion in Fig. 7, are more than satisfactory. Fig. 8 shows that scenario KG displays considerable overlap with scenario K except in the higher mass range. The plot reveals that the best fitting models for this scenario span a large range in Q$_{min}$ , and here some models (albeit very few) extend down to below Q = 1. However, Fig. 9 shows that all models for scenario KG are confined to X$_{min}$ < 2, unlike for scenario K where many models reside between X$_{min}$ = 2 and X$_{min}$ = 3. Also unlike scenario K, Q$_{min}$ does not appear to be significantly correlated with X$_{min}$ , and unlike scenario KL, in neither scenario K nor KG do we see X$_{min}$ drop below 1. As before, we selected several models from the PDF for scenario KG and evolved them forward in time. The results are found in the bottom panel of Fig. 10. As for scenario K, most of the available parameter space is bar unstable, with the low mass-high Q part of the plot susceptible only to mild bar formation. Thus, it is not possible to distinguish between runs K and KG on the basis of bar stability alone. Referring back to the rotation curve fits, we see that no models for scenario KG can reproduce the stellar rotation curve 17 5.2. Testing the rotation curve fits