Orange Free State Error Drie Pence?

Hoping someone could shed some light on a reference/any value to this stamp please?

This issue contains many errors and varieties.

My 2020 edition of the Stanley Gibbons Commonwealth and Empire Catalogue lists your example as "surcharge double, one diagonal".

Depending on whether the overprints are the regular or thick V, it could be either SG 117g or SG 128ba with catalogue values of £375 and £550, respectively.

To my eye, the horizontal V appears to be regular, whereas the diagonal V looks thicker. If you want to sell it, it may be worthwhile getting the stamp expertised.

Clive

The full-stop before the "I" is also higher on the diagonal overprint.

Should you wish to dive deep into this area, I present to you this reference, specifically Page 175 (warning, big file):

http://telstamps.org.uk/downloads/zzbookApart2.pdf

You can substitute the 2 with a 1 for the first part of this incredible reference. There is also an Orange Free State study circle if I recall correctly that could perhaps point to a specialist who could provide an expert opinion.