Czech 30 x 210 Headstamp


I have a Czech 30 x 210 steel cased round with the following headstamp:


The “Z” between the “72” is larger than the twpo number and appears to ahve been stamped later, as does the “OTK”.

Am I right in interpreting this headstamp as Lot 58 of 1972, made by aym, with “Z OTK” added later in manufacture. Are these some sort of inspector’s marks?

Thanks for any info.