Conrod end cap numbers

Hi,I need to replace one of my conrod endcaps but have noticed they have different numbers on the bottom,45,75 etc..I need to replace the 45 one but can't find a replacement  one with that number engraved anywhere!! What do these numbers mean?? And it is imperative to replace with the same number?? Sorry if this is a stupid question but thanks for any help anyone can give.