I agree with the writer above. I have had the opportunity to work with both the aftermarket and the OE and the OE is superior hands down. I just installed a new OE set in my 76, which is getting pretty extensivly restored, and they fit, well, like the originals, LOL. You will cry as you give them your credit card number, but in the long run it is well worth the money. If you still want to try the aftermarket units you can also get them from British Victoria.
MSA has been getting a good chunk of my salary for the last year. I just can't adequately identify all the necessary parts, working from their catalor (or Vic. British, either). I'd like to find someone who already knows what's needed.