Once again I’ve answered my own question, up to a point (pun intended). There is no reason why a Ball Point refill and a Rollerball refill can’t be interchangeable. Pen Kit manufacturers could (it seems) just forget Rollerball specific designs and just make Ball point pens then customers could just choose the type of refill they wish to use. I’m a big fan of this make (Schmidt) by the way.
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I’m guessing from the comment above that some pen manufacturers do indeed utilise the Parker standard for their Rollerballs.
Which seems like a wise idea. 