In America, Parker and Sheaffer dispensed with flexible nibs early on. It’s sometimes suggested that this was to suit multi-part carbon forms, but it doesn’t seem likely that whole brands would become stiff-nibbed for that reason. I suspect that it was much more to do with lifetime warranties. Even in heavy hands, a rigid nib will survive when a flexible nib may crack or become sprung. Where the market leaders led, others followed, and flexible nibs became relatively uncommon in the USA, though there were exceptions such as Waterman and Wahl who still presented flexible nibs in their range.
In Britain, lifetime warranties never became a selling point, so most British nibs have some flexibility. Again, there are exceptions, like the Swan Eternal nib, Conway Stewart’s Duro and most Summits and Mentmores. Newhaven Parkers, too, are rigid, but I think that’s down to the influence of the American parent company.
As interest in writing with a variable line grows, people are increasingly turning to older, traditional pens, as contemporary pen makers seem unable to make a truly flexible nib. Even among those older pens, unless you are able to try before you buy or see a writing sample, it’s something of a lottery. Most Swan SM100/60s have some flexibility; many are very flexible indeed. Some are nails, though. Similarly, late forties Waterman 515s can be superbly flexible or rigid. This is why I include an example of the pen’s writing in my eBay listings – to eliminate the guesswork.
In the pen groups and on some websites you’ll see horror tales of cracked and sprung nibs, and the suggestion is regularly made that writing with flex is almost inhumanly difficult. It’s true that accidents will happen, and over-exuberance can destroy a valuable nib. I’ve done it myself. On the other hand, I have super-flex pens that I’ve used for years without any problem. With a little cautious experimentation you can find the reasonable limit of your nib. Thereafter, you stay away from that limit. As regards difficulty: perfect copperplate is difficult; enhancing your usual writing with a variable stroke isn’t. Practice makes perfect and it doesn’t take very much, either, before you begin to fall naturally into the rhythm of light up-strokes and bolder down-strokes. It’s how people have always written, until the advent of the rigid nib and the ballpoint pen. The quill was flexible, as were steel dip nibs.
Indeed, if you want to try inexpensive and reliable flexibility, try a dip nib.
Excuse the scratching. I’m not at ease with a dip nib pen. In fact it’s not for me, with all that dipping and my unfortunate habit of digging into the paper with the steel nib! Good flex, though. This is a William Mitchell Panama No2 medium. I find it a bit of paper-cutter, but the Macniven and Cameron Waverley nib is more forgiving to a modern hand, in my experience.