FINDREP= must convert digits enclosed in separators. Otherwise also days, and/or years, and/or combinations of days/months/years could be converted to month names!
Enclosing in separators is not really required. If you notice the control statements above, only the month portion is OVERLAYed at pos-81 and the FINDREP scanning is set to start at pos-81 to convert the month(mm) to JAN, FEB, etc... The rest of the data would remain unaffected.
Not recommended, and non-flexible approach. Such methods, in general, are the main reasons why modern software requires monthly/weekly/daily/hourly updates, while old stable programs do work for years and years without problems.
The STARTPOS ENDPOS subparameters are there just to serve this very purpose. Ie. To limit the FIND REPLACE to specific positions. I don't see how it will be a non flexible approach. And old stable programs working for several years without any problems has nothing to do with this.