Hi Jack,
I have no insight into 230SLs, but early this year I was chatting to the Mercedes Heritage Archivist who happened to be visiting Mercedes World in Weybridge UK, while I was making a fool of myself in the new SLS on the test track.
I asked him about production numbers of RHD cars, but only in respect of the 250SL, which was only manufactured for 1 year. As he explained, every panel forward of the firewall is unique to RHD cars and in my case of the 250SL, where production was quite small, were often hand made at considerable cost. While there are no Mercedes sanctioned production figures for RHD W113's, the general view inside Mercedes is that only around 12% were RHD due to the very high costs of production. That means there were less than 300 RHD 250Sl's made.
The ratios may be similar for your 230SL, as the issue with the unique panels and the costs are likely to be similar., but this is still just a guess.
Jeff