Thanks for the various feedback - got busy on other things and didn't get back to this for a while.
The more I looked into this ATF+4 question the further into a wormhole I slipped. It does seem that ATF+4 was (uniquely) created for Chrysler sealed for life transmissions - perhaps it is over specified for the steering system out of convenience.
However - for the record.
ATF+4 is declared as Chrysler MS9602 standard.
The Mopar USA partnumber I could find is 05166226AA (473ml) and Mopar International 68218064GA (946ml). It is described as Power Steering Fluid +4 on these bottles or PSF+4.
I did find "Penrite ATF FS" $14.99/L at Supercheap and it's product detail PDF declares it is Chrysler MS9602 and ATF+4(R).
I didn't get into a Chrsler dealer yet to check the price for the real Mopar, but now I have the part numbers I am feeling braver.
If I wasn't being pedantic that should be good enough.
But just for the hell of it I have sent a query to the licencing body in the USA to see if they recognise Penrite as compliant / compatible.
www.centerforqa.com/chrysler-brands/
Just out of interest if anyone knows how much I am likely to need to drain and refill the power steering system it would be helpful.
And a link to, or where to go for the seal kit for the Merc box would also be much appreciated.