Complex sentence: Older guns still in production had their model numbers truncated, e.g. the Model 1912 shotgun became the Model 12, with one exception: the unconventional "Thumb Trigger" rifle, which was not previously numbered, was given the designation 99.
Split into simpler sentences: Older guns still in production had their model numbers truncated, e.g. the Model 1912 shotgun became the Model 12. There was one exception: the unconventional "Thumb Trigger" rifle, which was not previously numbered, was given the designation 99.