Help with finding mint marks

First of all, I'd like to disclose that I am extremely new at this whole "coin thing", so forgive me if this appears to be a silly question.

I have several baggies of Philippine coins that I am attempting to sort, and I've come to the 25 sentimos coins starting at 1975. The Standard Catalog of World Coins appears to be telling me that there are a few different mint marks for this denomination, but I can't locate where the mint marks are, at least on the 1975, 1976, and 1978 coins.

Can someone direct me to where I should be looking? Am I just blind? Thanks!

If you'll check and scrutinize your coin, you'll see a small letter "f" on the right side of the ribbon on the obverse side of the coin or tail. That means its was minted at Franklin Mint USA. Those without such mark are minted in Sheritt Mint Germany.

i'm a newbie too.. where could i find that mint mark from a 1903 five centavo and a 1935 five centavo heraldic eagle. thanks