"#" is for referring to a parameter defined in the class. Parameters are similar to static variables in Java.

https://docs.intersystems.com/irisforhealth20221/csp/docbook/DocBook.UI....

Hopefully this can save you some work. It uses a much larger chunk size (which is a multiple of 57) and works with or without CR/LFs (set the argument pAddCRLF):

Class Example.B64.Util Extends %RegisteredObject
{

/// Be cautious if changing CHUNKSIZE. Incorrect values could cause the resulting encoded data to be invalid.
/// It should always be a multiple of 57 and needs to be less than ~2.4MB when MAXSTRING is 3641144
Parameter CHUNKSIZE = 2097144;

ClassMethod B64EncodeStream(pStream As %Stream.Object, pAddCRLF As %Boolean = 0) As %Stream.Object
{
    set tEncodedStream=##class(%Stream.GlobalCharacter).%New()
    
    do pStream.Rewind()
    
    while ('pStream.AtEnd) {
        set tReadLen=..#CHUNKSIZE
        set tChunk=pStream.Read(.tReadLen)
        
        do tEncodedStream.Write($System.Encryption.Base64Encode(tChunk,'pAddCRLF))
        if (pAddCRLF && 'pStream.AtEnd) {
            do tEncodedStream.Write($c(13,10))
        }
    }
    
    do tEncodedStream.Rewind()
    
    quit tEncodedStream
}

}

My experience with Zebras was quite a few years ago, so this may or may not still apply... Using Zen reports at the time, the print server would end up rendering the label as a bitmap and sending that over to the Zebra. ZPL code to print an equivalent label was much smaller than a bitmap, so ZPL labels tended to print faster than those rendered by a report.