A new round of spy photos of the recently-spotted BMW Z-car prototype raise questions about its true identity. First believed to be a prototype for the SL-fighting Z9, it now appears this prototype test car actually be the next Z4.

Behold the first shots of BMW's new convertible prototype caught with its top down. It probably sounded like a good idea at the time, but the temptation of a top-down test run came back to bite these BMW engineers when our photographer caught the car stuck in traffic and pounced on them at a stop light. The close-ups didn't yield any meaningful dashboard details thanks to the interior's protective covering and the flailing arms of the engineers, but spies did manage to see some details that suggest this might actually be a Z4 after all, and not the Z9.

First off, we have been told that the MX-5 Miata you see driving in front of this prototype was no mere traffic coincidence. Our photographer reports that the Miata appeared to be part of the test group throughout the afternoon. It seemed to be on hand for comparison purposes–which would clearly point to the Z4 replacement as the likely I.D. of this prototype. These topless photos also reveal that this test car had cloth seats — something that would never be found on a car in the Z9's stratosphere — once again strengthening the Z4 case.

The stowed top also gives a more revealing look at the rear deck just behind the car's roll-hoops, and cut lines suggest that this could be a conventional soft top, and not the folding tin-top destined for the range-topping Z9. We can't know for sure until we see the folding top in action, but it all looks quite suspicious to us.

As for the apparent size discrepancy between this prototype and the Z4, it could largely due to some ingenious camouflage. A closer look at some of the space between the outer camouflage and the true body that lies beneath suggests that the camo shell may sit several inches above the real body panels in many places. The hood camouflage, for example, easily sits 4- to 6-inches above the true hood-line, at the cowl. Strategic placement of such misleading dimensional cues can be very effective in playing tricks on our eyes and perceptions. There may be a significantly more svelte and petit car under all of that armor. And the next-gen Z4 could logically be upsized a bit, if BMW plans to bring a smaller Z2 into its lineup by decade's-end, as rumors suggest.

These latest open-top shots of this Z-car prototype may raise more questions than they answer, but they give us a better sense of what lies at the heart of these mysterious test cars. Our first look into the cockpit area suggests that this is a car aimed at a more mainstream part of the market, which has us beginning to lean back toward the Z4.