[Milsurplus] SC-901X questions

[Hubert Miller]

What I do not get: this is an exciter, right, with like mebbe 2 watts output, yet it's packaged in its own airtight bulletproof cabinet.
I think that is odd. 
If it was used in missile silos, yes that would be low production, and with only that to go on, I'd W.A.G. maybe 500 units max ?
I don't think civil defense shelters, for civilians that is, had any radios. 
An aside: I saw surplused ( spellcheck is unfamiliar with this word ) ham gear around 1977 that had large "CD" decals on it. Like
'Harvey Wells Bandmaster" transmitters. Maybe some kind of ham network? But who retained the equipment, "officially" owned
it, til it was surplused ? 
-Hue 

>I had always figured that the SC-901X was a limited production set.  That is why I held onto this example and passed on all of the R-1051 receivers I had collected over the years.  

Have no idea just how many missile silos were equipped with this HF radio.  Perhaps it was used in other locations like civil defense bunkers and shelters?  A casual search
found that possibly a third example is in the SMECC collection.

My SC-901X is serial number 317.  How high do the serial numbers go?
Jim