Early prototype, eh? What happened to the flashbulbs on the backbox insert board, then? The large holes for them are there, but they're not. And I thought that prototypes had red lamp pcb's under the playfield, whatever their size. There is only one switch fitted to the upper stage of the Atomic Whip kickback, rather than the two implied by the double-length slot, and shown for prototypes.
Additional - the ipdb entry shows a prototype, where the drop target decals have a Blue background and a White skate. The playfield artwork also differs, on the 'scoreboard' showing the team names, a lightshield, and the 'design team' information. The display panel is a mixture, though. It has the Williams 'W' symbols on the speaker grilles, but the W-I-L-L-I-A-M-S spellout letters are yellow rather than the early games red.
At an asking price of £2,400, the sign lower left on the display panel probably applies; 'MUG (Root Beer)'