How to tell .303 magazines appart?

[Brizz]

I have a No.4 Mark 1 and a No.1 Mark III... The #3 doesn't have a magazine which I need, how can I tell the magazines apart? ie. some guy has some .303 mags but doesn't know what for?

 

[toyboy]

the easiest way to tell is the #4 mk1 mags have 1 raised notch in the rear of the mag to lock it into place.the #1 mk3 mags have 2 or 3 raised notches.
these mags are somewhat interchangable but YMMV(your mileage may vary)
some 4's will fit in 1's and vice-versa but not always.

 

[Vimy Ridge]

As stated by toyboy:

No1

No4

 

[Brizz]

Thanks guys!