The "thin 4" on a Mark 1 is always replaced. I asked my friend and Rolex watchmaker, if it is damaged the change it. Rolex had only "thin 4" to replace, they dont have the "fat four" inserts.
Rolex dont sale the green insert, it is only in change with the old damaged to buy. The price is about 300,- for a green in change, the price for a black one 50,- without change.
So it is very important not to damage the fat four insert, because this makes a diference from about 3000,- Euros in the moment. In Spain, i saw a Mark 1 for sale with "thin 4" F-Serie for 7000,- . Normaly the start with 10.000,- up to 25 for a mint Y-Serie.
Also you can not buy the so called "MAXI DIAL", they only change it and the dealer have to send the old one to Rolex, than they sen a new one, NOT MARK 1 and 2 with "5 Tics" .
Best Walter
Foto: my Mark 1, F-Serie, unpolished, Full Set