Here is what I believe to be another production engineering prototype of the Titanium 75. It is distinguished by the barrel end tassie which, like the cap tassie, is dished. This feature is like my other prototypes of this pen, shown as prototypes 15 and 29.
In these 2 close-ups showing the dished tassies on the cap and barrel, you might sense a greenish tint to this pen. Indeed it is actually on the pen and not caused by the lighting. In fact, I tried hard to have the camera record this hue so that you might see it. This effect was attained by heat treating the barrel.
Parker made many attempts to create a rainbow effect on the titanium caps and barrels. It was very difficult to do and they had very few successes. The best specimen is pictured as pen #11 on page 99 in the 1995 Lambrou book Fountain Pens of the World. Unfortunately I am unable to show this rainbow titanium 75 FP due to the copyright issue; you'll just have to get a copy of the book.
(If you know who is the owner of this rainbow titanium 75 FP from Lambrou's book, please email me. I will ask the owner for a picture of it so that everyone can see it on this website.)