The three letter prefixes show up in English (eg oct in octal, dec in decimal, etc.).

The prefix in "decimal" is "decim", like you'd expect given that the root is "decem". There is no "dec-" prefix.

(You do see "deca" used as a prefix, "a" included, but that doesn't come from Latin.)

You are right. I should have written: (septem)ber was the 7th month of the year, (octo)ber was the 8th, (novem)ber the ninth, and (decem)ber the tenth!