Complex sentence: The letters were arranged in Reichspost bags that the OSS had forged to resemble the original bags, these precision forged bags were indistinguishable from the real German mail bags and were mimicked down to the material used.
Split into simpler sentences: The letters were arranged in Reichspost bags that the OSS had forged to resemble the original bags. These precisely made bags were indistinguishable from the real German mail bags and were mimicked down to the material used.