I couldn't stand it, so I went and took off my FTn prism again and had a look. Yes, there are pins on both sides of the glass. So, the fingers should not be necessary to hold the prism on. This should mean that the FTn head will fit on bodies designed for the T or TN finder if the nameplate is removed.
I don't think it is a derailment. It is necessary info if the OP is going to track down a metered head that will work on his camera.
He needs to first take the eye level prism off and look at the rear notch to see if it is big or small. (For comparison, both of the ones I posted above are big notches.) If it is small, he needs the original Photomic head; nothing else will fit. If it is big, he needs the T or the TN head for a direct, no-hassle fit. And based on what I just saw, the FTn head will also work, if the nameplate is removed. But I have not tried that yet. (As I said, no need for me.)
When it comes down to picking which finder, I will opine that the original Photomic is the best. It is incident, reflected, or reflected with a narrow pattern. And it works on all Nikon F bodies, so the OP doesn't even have to consider what type of body he has. It is not TTL, though. I find that better, while others do not.
If a TTL finder is preferred, my first choice would be the TN finder. It is a center-weighted pattern, while the T is a simple averaging meter. And the only incompatibility is with early, non-modified Fs with the small notch. The FTn is center weighted as well, but it is also the most incompatible of all the F Photomic metering heads. You need a late F body to use it; if you don't have a late body, you need to modify your camera for the FTn head.
If you post a picture of the rear notch, Fragomeni, we can tell you whether or not it is the large one or the small one.