Marshall, there was considerable hand fitting done when Lugers were manufactured.
Switching out a breech block (toggle assembly) could end up changing headspace as well as the sear fit. If done by a competent Luger gunsmith, it would be fine...
Fine tune sites have a small screw that is turned by a very small spanner. Both the front and rear sights can come that way.
There would not be a rear site on the rear toggle of an Artillery...
Marc
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 Igitur si vis pacem, para bellum -
- Therefore if you want peace, prepare for war.
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