Left-handers hold their pens and cover the page differently to right-handers. Whereas someone who is right-handed will hold the pen under the line and pull the pen across the page, someone who is left handed will generally either hold the pen above the line with the page on its side, and pull the pen up the page, or hold the pen under the line and push the pen from left to right. Either way, the pen is held at a mirrored angle to someone who is right-handed.
The tipping material on most fountain pen nibs is ground to cater for right handers - a left handed fountain pen nib has the tip ground the opposite way to cater for the pen being held the opposite way.
We have medium point, left-handed nibs in standard size 5 and standard size 6, both of which are available in Bock's own nib housing and in "kit compatible" housings, which fit a wide range of commercially available pen kits for pen turners, including our own pen kits - please enquire if you are unsure which housing you require.