Well, seat belts were not required to be standard equipment in the US until 1968, and the first state to actually require their use was New York the end of 1984, so the lack of a law doesn't necessarily mean it was safe, but I agree it does imply a lack of concern.

However, the 1968 seat belt mandate included a mandate for seat back locks in two doors. A 1987 crash safety study (ref. DOT HS 807 067) was published comparing two door to equivalent four door models, specifically focusing on the seat back latch as a factor in injury/fatality statistics.

The findings were that in even moderate head-on collisions ~26mph with unrestrained adult rear seat passengers, the seat back latches in two doors were inadequate and in most cases would tear loose from the seat frame, so did not provide the perceived safety. Unrestrained adult rear seat passengers often ended up in the front at even lower speed impacts.
There appeared to be a benefit when the rear seat passengers were children who with their reduced mass were more likely to be contained within the rear compartment and not hurled into the windshield, or with adults in low speed frontal impacts ~10mph.

A totally separate question but still related to seat backs, how does one remove the seat back cushions? Does one remove the bottom to reveal fasteners below the back cushions? This car still has the plastic seat covers on just the front backs and they are a bit ratty with the corners and some seams split out, so was wondering what it took to remove those covers.

In my recently acquired '55 400 the front seat backs apparently have no latch mechanism. They are free to fall forward with heavy braking or by a rear seat passenger. Is that normal on these cars? It's not at all safe.

My intention is to install period-style lap belts but the seat backs remain a safety issue.

Interesting information. It sounds as though the '56 junior fenders, which don't have the brow over the headlight, nevertheless aren't interchangeable with the '55 Seniors due to the slight crease along the top which was omitted in '56.

What front fenders, irrespective of trim differences, are interchangeable with the '55 400? I assume all '55 seniors are the same but what about the '55 and '56 Clippers?

One trick to check for belt noise, with the engine idling and the belts tensioned properly (not too tight or loose), CAREFULLY apply a bar of soap to each side of the fan belt. This will usually quiet a noisy belt at least temporarily.

If it's not the belt, pour a can of water pump lube into the radiator. It will quiet a noisy bearing and prolong lifespan.

Ethanol and brass are listed as having excellent compatibility in the chemical compatibility database. Plastics are a mixed bag depending on what type of plastic. HDPE, PTFE, Nylon and PET are compatible. ABS, polycarbonate, LDPE, PVC, Tygon, Silicone, polyurethane are not.
Interestingly, ethanol is not kind to aluminum either.

In my recently acquired '55 400, the speedometer cable was broken at the trans and as it turns out it's due to the speedometer head turning very stiffly. It doesn't make noise, just takes considerable effort to turn, but does turn and operate, at least until the next cable breaks. I would think it should turn freely.

Any tips on speedometer clean and lube, which I assume requires removal?

My '55 Patrician is an odd combination of "features". It has full leather seats in two tone green, and headliner.

BTW, that seat leather is way thicker than anything found in vehicles today.

Meanwhile, the exterior is single tone white and the power windows and seats were deleted in favor of hand operated. Apparently somebody didn't trust all that new-fangled electric stuff.

Just to bring this to closure (hopefully), the transmission and rebuilt engine were installed and the car taken for it's first test drive. The direct drive clutch now works.

Just as a follow-up, I pulled the pump plate off the pump to discover that although the plate is '56 the pump is '55. That's an incompatible combination with the plate blocking the DC oil path.

I have no idea where that '56 plate came from but I assume it somehow got swapped when the guy rebuilt it, even though his only parts vehicle was a '55 clipper.

The transmission is now reassembled with a '55 plate and will be going back to the car with the expectation the problem will be solved.
We'll see...