OK, here are the extension housing seals and sizes, that were used in 904's>>>1966-1972: 2.254" OD, no dust boot, >> 1973-on: 2.375" OD>>also 1973-on 2.445" OD
Hope this helps. Your local transmission shop probably has these in their stock. Just a suggestion.
See my post. Always measure the OD of the seal. According to my parts catalog, boot seals (2.375 OD) used only on 1973-on, although most shops I dealt with used the non booted seal.