Technical NGK b4l or b4lm

I went to the parts store to purchase new plugs for Lumpy. I asked for NGK b4l's. When I got home I realized he gave me b4lm's. I called him and he says they are the same. I don't think so. Does anyone know the difference in them?

 

https://www.ngkntk.co.uk/wp-content/uploads/2011/03/2015-Horti-catalogue.pdf
Many others out there that will explain it all that’s the first one I found

 

The reach is shorter b4lm is 3/8". b4l is 7/16". I'm taking them back.

 

Maybe read that info.........

 

BI buy ngk b4l because according to my “parts professional” at O’Reilly’s this plug is slightly hotter than a Champion H-10 which is my “go to plug” for flathead ford v8 or equivalent to H-11(b-4l ngk). I thought all these plugs mentioned were 3/8 reach; am I right or wrong?

 

No John. The NGK websites says the B4L has a 7/16" reach. There is only .062 difference but that might be enough to cause a problem in certain applications.