Critique a possible buy, Arab/QH cross mare 11yo (pic heavy)

Here is the description about her:

11 yr old chestnut Half Arabian mare. HAHR *1A342438. Certificate of Enrollment International Arabian Breeders Sweepstakes. Gentle family horse. Excellent ground manners. Sweet as can be. Natural horsemanship training. Buddies up and follows without a lead.Passed our obstacle course with flying colors. Rides western, english or bare back in a halter. She's always been sound and has nice barefoot feet. Ridden around Bonsall, CA. Adoption only. Health care up to date, wormed 6/11/11. Teeth floated May 2011. This horse has been well cared for and has never been abused or missed a meal. $900. Owner can not afford to keep her. A percentage of the adoption price will go to the rescue.
We are using this mare in our Jr rider with teens. She's awesome. Our farrier hauled her here and hopped on for a ride bareback.
Rides in a D ring snaffle bit, semi quarter tree saddle. Alfalfa hay and no supplements. Barefoot and a easy keeper. No mare-ish behaviors. She is in a stall 15 ft from Cherokee. She does not show signs of heat.

They're only asking $900 which if she is all they say she is, is completely worth it (especially in the California market).

What do you think of her pictures? And also, what kind of tie down mechanism are they using? I don't think I've ever seen one where the reins thread through. What would the need for this be?

Thanks for any/all opinions!

A few more pictures of her.

I am not seeing a tie down mechanism at all. Looks like a plain old breastplate, no?

She is lovely. Looks to be pretty nicely built and the description is inviting. Seems well worth a look

I think it's actually a training aid to help the rider keep the reins in the correct position. I've never seen one before but that's what my friend said it looks like.

I really like that horse and would totally buy her