Just to report back that copy/pasting the diffs from this module when compared to 1.5.5a code, into 1.5.6b zencart files works well.
Obviously, copying the core files as they are is a no-no, and I imagine most people have already incorporated some changes, so merging is normal activity.
For the custom templates, I note that tpl_checkout_confirmation_default.php exists in the classic_responsive template (which I use), the other two files need to be copied over template_default template.
The admin/includes/classes/order.php file no longer exists, as we know, so that work simply falls away (store front class order.php does the job).
I did not try to update the HTML code to the standard of 1.5.6b, no doubt some improvement in column alignments or some such might happen, I might have time to tweak the code over the weekend, but this is just to give a heads up that the module absolutely still does work.
Thanks swguy!
Zen Cart 1.5.6c modified for Japanese language support. Upgraded incrementally from initial 1.5.5d. Currently planning direct upgrade to 2.1.0