Ok I did quite some reading about these babies, such as
here and
here.
As Max said, the 7621 has DiMarzio p/u's, the 7421 "just" a V7/V8 combo.
But, aparently, in '99/'00, the production RG7421s were frequently equipped with "leftover" necks from RG7621s. Pulling the neck and investigating the stamp is a give-away. I would bet that's what the stamp on above guitar would say.
The most telling visual difference between the RG7621 and the RG7421 besides the pickups is the p/u mounting: the former has triangular routing for the screws (DiMarzios), while the latter has square ones, as seen in the pictures above.
Also, the bridge base plate as well as the saddles seem different quality.
lp_dude_2 in this forum has a good comparison pic - left an RG7412, right an RG7621, note the pickup mounting:

Confusing instruments...:).
'07 Ibanez RG8470F FE, '03 Ibanez RG3120TW, '94 Ibanez S540CG
'07 EBMM Axis PBB; '10 PRS Cu24 LTD; '91(?) Lag Hardline; '94 Lag Rockline