Can anyone tell me what to look for to spot the difference between these 3 fixtures?
At first glance they all look the same to me (from pictures I've seen).

Some info can be found here (in Dutch), see p.11:
http://www.marcelstvmuseum.com/oude%20straatverlichting/philips%20openbare%20verlichting%201972.pdf

SRL en SRM seem identical whereas only the SRM offers one-lamp and two-lamp options (2x SOX 55W or 2x SOX 90W).
SRL (L as in Large?) supports by default from SOX 90W and up whereas the SRM (M as in Medium?) supports SOX 55W as well.

Both SRM and SRL have optics for a specific light pattern (broad pattern). The SRK has clear optics for a more uniform "deep" light distribution. The letter K in SRK perhaps stand for the German word for Clear (Klar)? Just guessing...

Neither have by default an integrated ballast tray (special order only); those that did have the gear try by default incorporated were labelled as SRS acc. Philips nomenclature (e.g. SRS 201) and came out later. I'm not sure about the Thorn MA50/90 models...

The K would be more likely to be K for Kurz (short)

That's what I initially thought too. But all have a length ("lengte") of either 1066mm or 1500m. Not exactly short

All of the lanterns have the same dimensions, so perhaps it's another of the lantern's parameters that it refers to. 

Perhaps, the third letter has no specific meaning at all; it may just have been chosen by random?

That's a distinct possibility, designed to confuzzle their customers... 

That's what I initially thought too. But all have a length ("lengte") of either 1066mm or 1500m. Not exactly short

Just read this again, that's a very long lantern.. 

From what I've seen, 3-letter model names from philips usually decode to First letter = lamp type; Second letter = Mounting; Third letter = Material.
However, it's clear the canopys are all the same material, and K,L,M don't appear as the third letter option on the reources available to me.
With that said, I have been told that the L is the smaller version of the M. And I have observed the K has a clear bowl, wheras the L & M have refrators. And for the lamp holders for lanters carrying 2x lamps, the K has one lamp holder at the entry end, one in the middle; the L & M have one at each end. And the bowl for the M is slightly shallower than the L.
I am probably repeating somethings already mentioned, but that's what I know.

Just read this again, that's a very long lantern.. 

Ah, damn keybord... Indeed 1500mm 

From what I've seen, 3-letter model names from philips usually decode to First letter = lamp type; Second letter = Mounting; Third letter = Material.
However, it's clear the canopys are all the same material, and K,L,M don't appear as the third letter option on the reources available to me.
With that said, I have been told that the L is the smaller version of the M. And I have observed the K has a clear bowl, wheras the L & M have refrators. And the for lamp holders for lanters carrying 2x lamps, the K has one lamp holder at the entry end, one in the middle; the L & M have one at each end. And the bowl for the M is slightly shallower than the L.
I am probably repeating somethings already mentioned, but that's what I know.

It never hurts to repeat this kind of information; thanks for that 

A good explanation, thank you.

AgentHalogen made a really resourceful comment on my Canadian MA60 / long boi SRS 876 post. It explains mostly every Philips fixture aside from the last letters on the SRM series luminaires. Interesting to finally figure out that those letters mean. This information needs to be archived an easily accessible! Perhaps a post discussing this under "Dating & Specifications" would be beneficial?

I just found a reference to the Philips naming scheme in Matthew Eagles' book "Why SOX Lights" - what he says is this:

The first letter would apparently be 'X' if the street light was of type SOX; 'S' was intended to indicate a SON or high-pressure sodium street light, although the SRS201 example mentioned later suggests this rule was not rigorously followed. The second letter denoted the type of mounting (post-top, lateral or side-entry, etc.) and the third letter indicated the type of material (sheet steel, plastic, aluminium).

Edit - Ah, I just read the post you linked to, sounds like that could be it.

I think the S for SOX got replaced with X in the mid 1980s as SON lighting became more popular. For example, the SRP / Goldfinger I and SRS fixtures that were designed in the 70s use the S for SOX, whereas later 80s fixtures such as the Goldeye and Goldfinger II use the X designation (XRP).