I think I'm missing something...

[Aer Fixus]

So, I've seen a lot of IBM keyboards, but this one doesn't look special to me...
It just looks like an ordinary grey label Model M... It's even at a higher price than the F AT that the seller has listed...

http://www.ebay.com/itm/271401772650

Where is the Mandolin crystal?

[Danule]

 Is it normal for it to have a speaker on the underside like that?

[lcs]

Is it normal for it to have a speaker on the underside like that?

yes, both my Ms have them.

[fohat.digs]

A 1987 will have the heavy plate and wire stabilizers. Prices on all these have been spiking. 6 months ago he would have gotten half that.

[JWK]

Is it normal for it to have a speaker on the underside like that?

yes, both my Ms have them.

Most Model M cases have a speaker grill on the bottom.  But I believe only the 1394540 and the 1394545 actually contained speakers. 

To the OP, yes, the keyboard in the ad is just a standard 1391401, the most common of the Model M keyboards.  It looks clean, but I don't see any reason for a high price - no mandolin crystal in evidence.   Perhaps just better than average salesmanship in the write up.  Or perhaps the date of manufacture was significant to the buyer.   

[DavidB]

What makes a 1391401 more valuable than another one besides condition?

[Aer Fixus]

A 1987 will have the heavy plate and wire stabilizers. Prices on all these have been spiking. 6 months ago he would have gotten half that.

Interesting on the thicker plate. When exactly did they change over, or did it change per factory?

This guy has also sold two others (silver label 86, and grey label 87) each for exactly $100.

This market confuses me. I can kind of understand SSKs going for sky high prices because people inaccurately view their rarity and want one like there's no tomorrow. i can also see the silver labels going that high on a good day. but eBay is filled with sub $50 (shipped, even) 101 Model Ms, many of which probably have the thicker back plates...

[semyorka]

What makes a 1391401 more valuable than another one besides condition?

Age, basically.  ~1989 and older were built a little tougher and heavier, so all other things being equal, they're a *little* more desirable to some people.  That said, the condition it's in is far more important.

[JWK]

What makes a 1391401 more valuable than another one besides condition?

I paid a little extra for one that was built on my birthday.

[maatmouse]

Looks like your industrial strength IBM Model M keyboard. The corner stone of the typing pool since about 1990 (at least in the UK). Solid, heavy, loud and reliable as hell.

Not sure about the grill on the bottom.