The 410 I sold earlier this year had a tow bar fitting behind the bumper, there was a cross bar between the main bumper bolts, an aluminium spacer behind and in front of the bumper and the tow ball was actually bolted through the bumper itself. Behind all this the cross bar had two braces one each side bolted to the boot floor. I do not know if this was a tow bar manufacturers kit or specially made but there is a 410 near me which apparently also has a tow bar hidden behind the bumper, it has the tell tale blanking bolts in the bumper so may well have had exactly the same set up.
This means obviously that it is only the rear body structure and floor that takes the strain and is probably fine if everything is in good condition.
My 603 has a tow bar but this is a different proposition altogether and was custom built by the previous owner, on this the cross bar picks up on the mountings for the rear bumper but sits below it as does the tow ball,the bars that go under the car are much more substantial and while they are bolted to the boot floor they extend much further under the car being finally bolted to the fixings for the main suspension units on the chassis, so in this set up the pull is also from the chassis not just the rear bodywork/structure.
In terms of tow bars for a Bristol the only company I am aware of who can supply them are Watling who list bars for an amazing range of classic cars and supplied me with an excellent and very strong set up for an S2 Bentley.
On the front of the 603 the bumper mountings look substantial but the problem like the 412 is they are part of the body and are not therefore the ideal point to attach a tow rope, possibly if it was linked round both mountings it would spread the load but I would not risk it, a fitting direct to the chassis would be much better.
I suspect we will end up welding a towing eye or shackle mounts to the front cross member or as you suggest a threaded mounting so that the towing eye can simply be fitted when needed and would be a neater solution..
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