I've had similar experiences. I have a 1970s-vintage Canon AE-1 -- an amazing piece of equipment. Back when it was newer, I took the receipt with me when I left the US. I seem to recall I had a customs guy tell me that the oval-shaped "Passed" stickers indicated that the item had been imported into the US.

More recently, I returned from SYD to SFO with a brand new lens for this camera. Combined with the loose opals I had bought for Mrs. Flies, I was way over the $400 limit in effect a few years ago. I went to the Red line and got two surprises: loose gems aren't subject to duty; and, only digital camera equipment purchased outside the US is subject to duty. On this particular trip, I didn't even bring the receipt for the camera with me.