Here is a prototype of the titanium 75. I believe this to be one made by Parker's production engineering department to test the feasibility of using the T-1 parts to make a 75. Note the concentric lines near the tassie end of the barrel just after a smooth area. It is almost as though the intention was to allow posting the cap without causing surface scratches to the barrel.
Also note from the above that this unit has a barrel end tassie! This is unlike the production version which retained the red plastic jewel from the T1. This might mean Parker tried putting a barrel end tassie but later just decided not to bother with this extra step and simply use the T1 barrel as-is.
Other interesting differences can be seen in the picture of the cap.
Obviously the tassie is flat, quite unlike the production version. (The barrel end tassie on this prototype is also flat.) Also notice that the clip is the early style which has the smaller arrowhead and shorter feather. The production version used the later clips since that was what Parker had started to use on the 75s around the time of the T-1's demise.
I think this gives us a good idea of when these early clip and flat tassies were replaced by the later ones -- 1972. In fact this is theory consistent with the last appearance of this type of trim in their National Geographic ads. By the time they placed the ad for the vermeil 75 in the August 1972 issue, it had the later clip with the larger arrowhead.
From all these observations, I would speculate that this prototype was made early in 1970 or 1971 when the T1 was first being introduced and sold. This was not something Parker did after they decided to kill the T-1. Why? Because by that time the clip and tassies for the 75 had already been changed.
Another prototype variant I found had an entirely different cap tassie and shown below.
This is prototype #25 and you'll notice that the clip arrowhead and feathers indicate that this is the later version normally paired with a dished tassie. But in the circular depression, a red plastic jewel has been glued into place. This colored plastic, which semi-spherical, is reminiscent in its color to the flat jewel found on the T-1 FP.
The weight of the cap and barrel are 4.5 and 4.8 grams, respectively, for a total of 9.3 grams. This makes it the second lightest 75 FP after another titanium variation.
Other variants of the Titanium 75 FP can be found in #29 , #38, and #39.