I have a mod 29-5 with 6" barrel. All references that I can find in the Blue Book show only 4,6 1/2, & 8 3/8. The barrel length is printed on the box as well.
After around M29-3, they changed from 6.5" to 6". Then they switched back to 6.5", around M29-8 or something i don't remember. Your M29-5 should be 6".