Thanks for the clarification, Craig. I haven't previously seen any import marks from that cycle and hadn't realised that the punch shape was different to the standard one. It doesn't appear to be mentioned either in Jackson or on the Assay Office web site either. However, now that you've pointed it out, the difference in the shapes of the "S" is apparent.
The use of the scales convention mark is optional and does not seem to have been used very much until relatively recently.