What does this mean on my accutrn watch has a case code of B26458 and M7 ?

Step 1: Find the battery code: In order to identify the correct replacement battery for your watch you will need to know the battery code. Sometimes this is printed on the back of the watch - as with some Timex watches - but in many cases you will need to open your watch case to see the code on the battery.

Most likely a 370,371 or LR44

This is a 1967 watch. The battery was a 329.

http://www.mybulova.com/forums/accutron-spaceview-1967

The 329 may come in two sizes,one is high drain, the other low drain. That means there is a question of fit. However, Bulova documentqtion indicates that the places that sell Bulova watches can supply batteries. You might check the Bulova website for a retailer near you and give that a go. Alternatively, below is a link to 329 batteries.

https://www.amazon.com/Renata-329-Button-watch-battery/dp/B001C17ZZY


Parts Manuals

https://www.batterybob.com/products/32-accucell-1-343-diy-replacement-kit/

Yes, Bulova used M0 to indicate the year 1960, but the marking on the case may not necessarily match the movement. It is not uncommon to find a movement that was produced one year earlier or one year later in a case made for the model. So, although the case of your watch was produced in 1960, the movement may be from 1959 to 1961.

M8 in the serial number of your watch signifies that it was manufactured in 1968. Here is a list of codes that are used to identify the year the watch was manufactured just in case you were curious.
A8 = 1948
A9 = 1949
L1=1951
L2=1952
L3=1953
L4=1954
L5=1955
L6=1956
L7=1957
L8=1958
L9=1959
M0=1960
M1=1961
M2=1962
M3=1963
M4=1964
M5=1965
M6=1966
M7=1967
M8=1968
M9=1969
N0=1970
N1=1971
N2=1972
N3=1973
N4=1974
N5=1975
N6=1976
N7=1977
N8=1978
N9=1979
P0=1980
P1=1981
P2=1982
P3=1983
P4=1984
P5=1985
P6=1986
P7=1987
P8=1988
P9=1989
T0=1990
T1=1991
T2=1992
T3=1993
T4=1994
T5=1995
T6=1996
T7=1997
T8=1998
T9=1999

You need a special tool called a case opener, if you flip the watch over you should see 6 square slots on the outer rim, this means that the back case is screwed on and probably has a water proof seal, keep in mind there are different sizes. I suggest you check ebay for watch tools, you might find what you are looking for.

If you are looking to purchase a new watch, you have likely seen that there is a measurement for the watch case. This does not mean the box which contains the watch. The watch case is the space between the two arms that extend above and below the watch where the band attaches. The end of the band will fit in between the watch case. You will need to know this measurement both in purchasing a new watch and in looking for a replacement band. Often, watch makers give this number in millimeters, and for that reason, you will need to use a metric ruler to make the measurement.
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1. Find the watch case. It is the space where the end of the band attaches to the watch face. Refer to the image for a picture of the location of the watch case.
   
2. Using the metric ruler, measure the distance from the inside of one of the arms to the inside of the other arm. Watch makers use the inside measurement when creating their watches, not the distance from the outer edges of the arms.
   
3. Write down the measurement on the piece of paper. Measure the other watch case, but they should both be the same measurement.

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There may be three number on the back of a watch.
1) The case number.
2) the movement number.
3) the serial number for the case +watch assembly.

If you go to http://www.bulova.com/service/servicecenter.aspx, you will be able to locate the nearest service center, who will be able to advise you. (incidentally, after such a long time idle, it may need servicing).

Of course, there is the way to change the battery, but for this you have to visit your closest watch repair shop. I do mean WATCH REPAIRS, not the jewellers. Decent watchmaker will definitely open your watch and will replace the battery. Your watch's case appears to be a monoblock. That means you'll need to lift the crystal first, pull the stem second and just then you'll be able to extract watch's movement for replacing battery. Do not forget: jewellers and watchmakers are under one roof, but their trades are completely different. No jeweller can do watchmakers job and vice versa. Rate me, plz.

First -let's check out what type of watch you have. If you push the crown in and it turns freely back and forth with no resistance at all - this is battery driven watch.
If you do the same and feel and hear some clicking and resistance- this is mechanical (spring driven) watch.
Next - the case: Usually all pocket watches have snap-on casebacks. Take an eyeglass and examine the case. Even if you think there is no any grooves or gaps- try to pry it using watchmakers caseback opener (about $5 on ebay).
In case if there are no chance to open caseback - that means you have one of the cases assemblad watch glass fitted last and that means you have to take glass out first and then only to get to the movement.
As you will not be able to get the glass out without braking it, the only solution will be to visit your nearest watchmakers shop and ask for help.

Rate, please.