Kai, I'm only talking about Javanese keris here.
The style of the ron dha is not the choice of the maker. Each classification of keris has a different style of ron dha.
How the maker cuts that ron dha is the product of his hand and mind. He may not vary outside very strict parameters; he must conform to the prescribed style, his only latitude, if there is any latitude is in the way he makes the shape. Makers did not "decide to cut a different ron dha". It was never their choice.
It could possibly be likened to telling 100 different people from the same societal group to draw a circle, a square, and an isosceles triangle. The figures they have been told to draw are all drawn free hand, they must not be larger, nor smaller than a particular designated size, and they must all be placed within a space that has been preset.
Those three figures drawn by 100 different people will all look the same at first glance, but as you study them in detail you will find very small variations in the form, variations that have been caused by the physical form and abilities of the individual person who has done the drawing. Some of these miniscule variations may be able to be altered, other miniscule variations will not be able to be altered. It is the unalterable variations that say who drew the figures, similarly, it is the unalterable variations that say who cut a ron dha.
Have a look at page 165 of "Keris Jawa".
You will find a number of examples of greneng in the Kartosuro/Surakarta style.
These are complete greneng, and looking at them you could easily be influenced to believe that each of these empu cut exactly the same greneng each and every time he made a keris. Nothing could be further from the truth. The fact is that the complete greneng is made according to the available space, the request of the person who placed the order, the requisite iconic content, the requisite talismanic content --- and maybe some other factors. These things can vary for each keris, but the ron dha is the constant component and is virtually invariable.
Now, keep looking at P. 165.
There are six examples of greneng in the same style by six different makers. At first glance they all look the same. Now look carefully at each ron dha:- there is enormous variation ; no two are even close to the same.
Kai, this is the essence of Javanese keris study. You do just look at the entire keris, or even a part of the entire keris. You examine it in microscopic detail.
Now let's fast forward to post 1970.
Could a talented maker decide to copy the ron dha of somebody else?
Yes, I guess so, but why would he want to?
He cannot produce an entire keris that is worth any serious money, in the style of one of the greats. I'm not talking about the type of thing that is worth a few hundred dollars, or even a few thousand dollars, I'm talking about serious keris that are worth serious money. To my knowledge nobody has yet displayed the necessary talent to do this, at least not at a sufficient level to dupe anybody who does understand keris.
If he copies somebody else's ron dha, where is the profit? Nobody wastes their time for no return.
Incidentally, for those with the time and level of interest, here is a pretty good site. It is in English and Indonesian. It may give a slightly better understanding of the current Javanese world view.
http://www.jagadkejawen.com/