Vallejo Dunkelgelb Surface Primer

[Sir Rodney Ffing]

I sense I may well be missing something very obvious with this question, but I'm stumped so here goes.

So time ago I bought the Vallejo Dunkelgelb German Dark Yellow box set with a step by step guide showing the order to use the colours (which were unexpectedly in those puny 8ml bottles, but that is another story). The base coat is Surface Primer #604 (labelled on the bottle as Dunkelgelb RAL7028), which is best described as a darkish sand colour and works very well with an olive green pattern. Step 2 involves applying Model Air #025 (also labelled Dunkelgelb with the same RAL number as #604), which is a darker, more yellowy shade, to the recessed areas. 

After an unfortunate accident when the top came off the bottle, I lost about half of the remaining amount of #604 and tried to buy some more. Lots more. Enter a 200ml bottle "Surface Primer 74.604 German Dark Yellow Dunkelgelb RAL 7028", which is very close in shade to Model Air #025 but a much darker yellow than the Surface Primer #604 I was after. No matter how hard I shake the bottles, the colours are nowhere near a match.

Can someone explain what is going on here and why Surface Primer 74.604 and Surface Primer #604 are so different in shade?  Also, is #604 available separately, or only as part of a box set?  My Google Fu must be very weak today as I can't find answers to either question.   

Thanks for taking the time.