I have seen the odd variation of a Parker 25 that was not a genuine production model. These are said to be specials made up by request, or quite likely 'frankenpens' made up from parts.
As the Parker 25 is made up of very few parts. If you have some knowledge of pen restoration it is not too difficult to disassemble and make up any combination you desire.
Some variations look appealing, although I feel not being genuine takes away their element of authenticity.
Below: Matt black Parker 25B FP with orange trim, flat top, Mk1 nib and date code.
This would not have been a 'special'. The orange parts were manufactured before the matt black was introduced therefore it is not genuine. I made this pen as an example.
Below: Matt black Parker 25B FP with green trim, flat top and date code. This combination might have been possible.
Above: A stainless steel 25 FP with an 18k Sonnet nib replacing the original steel nib.The owner states "The pen is a very nice writer. I like the robustness of the P25". Courtesy of Robb Swenson, Chanhassen Minnesota, USA.
Above: Parker personalization kit. This allowed the owner to stick a letter on the top of a 25 cap. A perfect fit in the ridge / dimple top version. Photo courtsey of T.Illingworth FPN