Going off of your info, W301970 I have one source that says it is a is #16 o-ring, but Ford Parts Giant says it is #17.
-W301970 - Genuine Ford Base No. #HM1 Hardware - Miscellaneous
Ford parts #W301970, Base parts number #HM1, $5.25 online at FordPartsGiant.com. Wholesale-priced and fast-delivered Ford parts Hardware - Miscellaneous.
Hardware Miscellaneous W301970- | TascaParts.com
W301970- - Hardware Miscellaneous