Stupid question Re: grid references

[U99]

Hi all,

I've done a few patrols but there is one thing that still baffles me, and it's probably something really stupid but I can't work it out

the KM grid is marked out with each square called usually two letters and two numbers, such as BF15

now... whenever I get a radio contact report from another U-Boot, it always quotes two letters and four numbers, for example AM2538.

so... where is grind AM2538? what's the convention for working out what these references actually mean?

thanks