What do factory markings on the inside of DUPLO pieces mean? In addition to LEGO and DUPLO names embossed inside the pieces of early DUPLO figures, there are a variety of number formats (one to three digits, some hyphenated) and in the case of 1977, the year.

Is there an explanation for what these numbers mean - and can they then be associated with a particular set and/or year?
The markings are the Batch number and element id.