Hi JBeL,
RE: "I've seen these dustcaps on the Sentry iii series 1 model all be it with accordion surrounds on the 15".
RE: "... dosen't have a pic of the woofer as in the ones l'm looking at though"
An accordion surround would seem surprising here: unless another driver used or a Frankenstein woofer...
Below is a pic of what an original Sentry III (first edition) should look like. Though the pic is small, woofer appears to have a foam surround and no "EV" indication on the dustcap. That should be the reference.
On the DL15W foam surround woofer spec sheet, the driver pic shows maybe half the dustcap and no "EV" indication in sight. Yours could be a DL15W with another dustcap or even an oem EV driver made specifically for that Sentry III.
When refoaming surrounds, some folks do cutoff the dustcaps in order to shim so the cone doesn't move and keeps the right gap. Then the person doing the job maybe had on-hand proper size dustcaps with the "EV" on them as replacements, so put these on... I think the above could be the likely explanations.
The Sentry III original pic here also reveals another "difference" with your picture (under the horn tweeter). The vent is a two-part one, but on your pic, contrary to the original pic, the lower part of it is blocked by the port cover, meaning the box is in low-frequency step-down mode, and normally it shouldn't be with cover on, unless one has or gets the EV SEQ equalizer that goes with this.
EV doesn't recommend the use of its speakers in step-down mode (port cover on) without EQ use. As I recall, the normal box tuning here is around 40 hz and the step-down tuning is around 28 hz.
Simply stated, downtuning a box puts more strain on a woofer (having to reproduce lower frequencies), which the EQ normally "compensates" for this, and if you have that port cover on with no EQ, the bass will sound weaker since the woofer now has to reach lower, therefore work harder (more excursion) to deliver a higher VLF level you may want to achieve. If you don't get the SEQ with the speakers, its better to remove the port cover and leave it that way.
Btw in another forum I was reading a fellow's comments on the Sentry III sounding better than the Sentry IV. Well, that should usually be the case considering the Sentry III is a monitor type loudspeaker, whereas the Sentry IV (all horn loaded, including LF) is a Sound Reinforcement loudspeaker system engineered for max output. Two different animals for different purposes. I guess the fellow didn't realize the Sentry IV isn't an updated/improvement of the Sentry III, so comparing different beasts... Regards,
Richard