Just had an email back from erg, see below.

Thank you for the inquiry. In short, to the best of our knowledge, this inverter is no longer available. This is a part that was produced, under license from us back in the 1980s by a UK firm (long before my time here). However, as far as we can tell, that company no longer exists. Also, we did not produce an equivalent part here.

Apologies that we could not be more helpful.

fairly conclusive id say.