Difference between red 25 and 25A filters?

[Tony-S]

I came across a red 25 filter and it is noticeably darker than my 25A filter. I couldn't find a ref in my books that discussed the differences, so I'm hoping someone here might know. I'd say it's a half stop darker.

 

[Lee L]

(there was a url link here which no longer exists)

 

[Tony-S]

OK, thanks (and thanks to those who PM'd me). I'll give each filter a try on the same roll/scene and see what differences they have.