I have already discussed it with the machinist who makes the extensions I sell now and there are other considerations. If the extension was made to provide for another round, the magazine spring would also need to be replaced, the cost for the extension and new spring would probably be in the seventy plus dollar range. There would be a lot more machining for an extension with a recess for the extra round, finding someone to make a small amount of springs, all adds up to more work and cost than most would be willing to pay for. IMO
John