Dear Daniel,

I agree that

          We talk of mnemonics.
          And, NORMALLY, nobody works thinking in the opcodes.

BUT, what actually matters is what is burnt in side the mcu (it is the opcode) that governs the flow of program.
That is why I suggested to check opcode generated (Is it harmful to cross check?)

Also you have said,

      in all cross assemblers for 8051 I know,
      the structure of moving one internal data value into another is;
      MOV dest, src for the mnemonic.

correct, I also came across many assemblers and they are converting this mnemonic as you have mensioned. But still I cannot Guarantee that each and every assembler will convert it in same fasion (remember I had asked him to check manual of assemblers he is using. also I had suggested to check .lst or equivalent file)

Regards,
Mahesh