[mrerick]

A much easier (and possibly less expensive alternative) that will not alter your existing collectible Erfurt Luger would be to buy a complete cannon assembly. This is the barrel with sight attached to a receiver. The complete barrel and receiver with sear.

This might require fitting since there could be slight differences between the fitment of the sear on the replacement, and the complete trigger and side plate. Obviously, you would only make adjustments on the cannon not the other parts (which would still need to work with the original Erfurt cannon assembly and it's sear).

The cost of the complete upper (cannon) with a properly fitted toggle train might be less than having a barrel fit to the Erfurt receiver. That would be much less reversible since it would require a gunsmith again to put it back into original condition - something that could not be achieved easily.