Can the fiber optic rod that fits the front sight base be purchased as a replacement item? Mine is now broken...

Don't believe so.

Check with Brownell's. They sell replacement rod inserts.

I just installed a new front Williams Firefly sight on my Marlin 336, the entire unit.

yep BT/DT

I called Williams and they are sending me a piece of fiber optic rod, free. Brownells sells replacement, it just must be bought in bulk.

actually Williams did the same for me and I got a spare just incase