What is the estimated value of a milton upright piano serial number 51839

It appears to be made in 1910. There are so many questions that would determine the value. The condition of the finish, the condition of the interior mechanism, the scarcity, or over abundance, of pianos in your region. Are the keys ivory? How's the sound? In general, old uprights don't sell for much. However, if it's got a great sound and is in really good condition, it can sell up to $1,000

Milton Piano
From http://antiquepianoshop.com/online-museum/milton/ :
The Milton Piano Company was established in New York in 1892. In the early 20th Century they offered a full line of uprights, player pianos, and grand pianos, and they enjoyed a reputation of building very well made instruments. In the mid 1920s, Milton built a coveted new state-of-the-art factory on West 51st Street, New York City. There is some evidence that Brambach and Milton were somehow affiliated with one another, and their factories were located within blocks of each other. The Milton Piano Company built pianos until the late 1950s era.
From http://bluebookofpianos.com/agesm.htm#MILTON :
This name is a familiar one to the magical world generally. Milton pianos have been manufactured for many years and always with the care that produces most satisfactory results. The modern factories in which the Milton piano is produced are new, large structures located at 626-630 West 51st St., New York City. Milton pianos and player-pianos are thoroughly well-made, beautiful instruments, designed for a class of discriminating music lovers. They possess a tone at once powerful and sweet. They are pianos in which purchasers are assured good values and they are pianos that give exceptional satisfaction. Milton reproducing pianos are equally representative and popular.
All of which is great, but:
Be aware when dealing with antique pianos:
Many of the old (say, 1850 to 1940) pianos were made by companies that are no longer in business (failed, or bought out by a competitor), or companies that built the instrument under another name.
Pianos back then were not really mass-produced, they were hand crafted- and each manufacturer made their piano very differently, so:
Short Version: there will be parts that will work on only that *one* model of piano, so getting parts will be difficult if not impossible.
Long Version: Musical instrument craftsmen (craftspersons?) love to innovate, for a variety of reasons: they may find a new way, for instance, to connect the key to the hammer that strikes the corresponding note. It might be a better way to do it, or, more often, it was to come up with a cheaper way to do it.
The point I'm making is that your Struz Brothers piano will have parts that are totally different from, say, a piano made by either Krakauer Brothers, Doll, Jacob & Sons, or Mathushek & Son Piano Company (companies that started around the same time as Sturz Brothers). The parts may vary from one year to the next, or even one model to another.
So, if your Sturz Brothers piano needs a key replaced, or the harp has bent under pressure, the soundboard is cracked, or any number of things that can plague an old instrument, you'll need to either
A) find someone that is willing to sell parts from a similar model (if they're parting it out, it has more problems than yours does), OR
B) you'll need to have the part manufactured from scratch by craftsmen that specialize is rejuvenating old pianos. This will be expensive far, far beyond what the piano is worth- and these craftsmen live very well, BTW...
Last Caveat: Upright models sell for much less (and the term "Upright Grand" was just an advertising phrase, there is no such animal.) Uprights (spinets particularly) sound very... tinny, is the best description I can think of. Tinny like the toy piano your granma gave you when you were five- remember, the black keys were painted on? Tinny like that.
So, unless the piano is a Steinway & Sons, a B?sendorfer, or a Fazioli, don't count on huge offers for your instrument. And if it came to you from your great-aunt Tilly and you can't bear to part with it, that's great, I understand sentimental value. Just know that this will cost a great deal to bring to playing condition.
I'm often told "But the keys all work, the pedals are still tight, and the tuner told me that it's not a bad piano!" All this is true. But please remember that professional buyers know all of these potential pitfalls won't offer much, and the guy on Craigslist that needs to replace the piano for Great-Aunt Tilly can't afford much.
Good luck!