A little white fingernail polish on top of the holder and a standard ball point pin will allow you to mark the finished crystal with the frequency.  Pilot G-2 series size  07 pin works great. Finish with Clear polish.  Buy your polish at the Dollar Store. BTW  May 1963 "QST" has a nice article on raising and lowering HC-6 crystals once they are out of the holder.   Very easy to raise in frequency but moving lower in frequency is difficult - the process is not very precise but allows moving those HC-6's that are out of the ham bands. Be prepared for fatalities.
Z