I did a search for caches in the area and didn't find an geocaches listed on Healy, Rd. Since the log book is empty it's possible that the geocache is not yet published. How long ago did your letterbox go missing? If it's been more than a few days it could be a cache that was placed (I *really* hope someone didn't try placing a cache there and decided that your letterbox was "in the way"), the reviewer would not publish it, and the person that put it there hasn't bother to retrieve the geocache. Since it's on Conservation Land Trust property it very well could be that the reviewer denied publishing it because typically geocaches are not allowed to be placed on conservation land trusts.
The cache name is also leads me to think that it might be part of a series (TBWS #1, #2, etc) and there is another small series nearby. You might even find that if you searched a bit about 528' down the road you might find TBWS #4 or TBWS #6 (half-joking...).
I'd do a search using the Hide and Seek a Cache page (My link) in a couple of days and see if any new geocaches show up. Then you can send the cache owner a message and ask what's up? If no new caches show up you can also send an email message to the local reviewer. Here's a link for what appears to be the local reviewer: Massechusets Volunteer Reviewer. They would know if someone tried to have a cache published at the location of your letterbox.
I would suspect that most geocachers are aware of Letterboxing, and in fact there is a letterbox hybrid geocache tpe (though they're not very common). However, a geocacher should never, ever, remove an existing letterbox or even another geocache in order to place their own geocache in the same location. I hope that this incident does not sour your opinion of geocaching.
Madmin Very nice guy to deal with. Both of my caches are in Mass. I am sure he will help you out, if he can.
*whisper* MadMin is a she.
Ya know, I thought about this last night after I posted it, I should have put person, because I did NOT know .
PS if you are MadMin I am sorry.....
No worries! I got a good chuckle out of that []