A FAQ on the weather helm issue had been collated in the period approximately from 2000-2015, and which was put here in the process of launching this website in 2019. However we recognised that it could usefully be improved and so we started a renewed effort to understand the extent of the weather helm issue, and the the available fixes. This has now been achieved.
In summary weather helm affects the unmodified mk1 cutter. It is greatly or fully resolved in the modified mk1 cutters, depending on which 'fix' was applied. It does not affect the mk2 cutters, nor any of the ketches. Furthermore it seems that reefing the mainsail in a mk1 at the correct windspeed to achieve maximum sailing performance also fully resolves the weather helm issue (this was demonstrated in the VPP analysis, see below). This explains why some unmodified mk1 owners had never encountered the weather helm issue at all, i.e. no modifications are actually needed if they are sailed a particular way, and that way happens to also to deliver the maximum speeds.
The initial FAQ from 2000-2015 was updated by all owners in 2019-2020. It resulted in a much improved document including all the Anecdotal Weather Helm Evidence gathered from owners (which includes all the original FAQ) last updated in Feb-2020. The report is one of the documents in the Longform Articles section of the website.
Following this a theoretical study was carried out in April-2020, with the results written up in the Theoretical Weather Helm Analysis that aligned well with the anecdotal reports, but extended and improved the understanding of the causes, and the available solutions. The full report and supporting documents can be downloaded from the Longform Articles section of the website.
Subsequently further theoretical studies were carried out into static stability and dynamic stability and seaworthiness, also available in the Longform Articles section of the website. An aspect that was studied during this was the actual sailing performance of the different Corbin 39 rigs, using a Velocity Prediction Program (VPP).  This was written up in June-2020 as Theoretical Analysis – Sailing Performance & Velocity Prediction Programs (VPP).  During the work on this it was realised that the optimal reefing windspeed for a mk1 cutter coincided very well with the point at which an unreefed mk1 cutter would begin to experience the onset of significant weather helm. This is discussed more in the report, and armed with this knowledge skippers can extract maximum performance from their boats.  The full report and supporting documents can be downloaded from the Longform Articles section of the website.
As an aside it is now understood why the boats that made changes to the forward edge of the skeg would not have achieved any real benefit from this, however we have not had an opportunity to write that up - it is something of a blind alley in design terms and there is always other work to do. The reported changes to the skeg would have done no harm either. (remark, DS, June-2020).
If more information comes to light then we will update this FAQ etc. If you have more information please contact us.