Difference between these two master cylinders?

What is the difference between these two master cylinders? Dorman M75817 and M75818.
Per the summit page M75817 is for manual brakes and
M75818 is for power brakes. Or at least it is the correct part if you have a pdb b- or e-body. 817 is listed for disc brake cars only if they are manuals (a-bodies).

From the outside they look the same, the piston diameter is 1.031 on both, and also both have reservoirs for disc brakes. So where is the difference? Can I use M75817 on a power disc car (1971 Super Bee in my case)?

For 1970, a B body with power or manual disk brakes would have the line connections inboard.
http://www.imperialclub.org/Repair/Lit/Master/274/page13.htm

If that's true for '71, then I'd say they are both wrong for your application. (edit: 1971 is different)

Shop manual will have bore and stroke. My notes say the '70 b-body master was 1.125" bore. Here's a chart Moparts member "Mastershake" created for a Mopar Action article.
http://www.moparts.org/Tech/Archive/brake/14.html
I know he's made some corrections since then but it looks like the correct master for '71 will have a 1 1/32 bore. Do a some searches on the p/n 3461176 and casting number Mastershake has posted. Then you can compare with the Dorman substitutes.

1971 is outboard. Both of the mentioned parts are outboard.
817 is for manual brakes, 818 for power (disc) brakes. Both look identical, have the same piston diameter and reservoirs for disc brakes. But where is the difference?
From the Parts Catalog I need 818 (replaces orig part #3461176) for power disc brakes. Want to convert my brakes from manual drums to pdb. Already bought 817 long ago and wonder if I can use it for pdb.

If you don't get a response in a few days, try PM to Mastershake - he may not look in this forum as much as Restoration and General Q&A.

If the photos of the Dorman's are accurate, yes the large resevoir indicates both should be for Front Disk, Rear Drum.
Generally the drum brakes will have a residual valve in the master cylinder outlet. http://www.imperialclub.org/Repair/Lit/Master/274/page04.htm This is because the springs on the shoes force the brake fluid back into the m/c. (Some later models moved the location of those valves)

According the Mopar Action chart, '71 B-body disk system used the same master cylinder for Power and Manual.

I can see why it seems from the Summit info that they are the same; something there may be wrong.

Need a '71 Service manual to see if there was any differences in Bore and Stroke inside the Drum M/C to Disk M/C for your Bee. If not, what aftermarket sometimes does is sell Drum brake MC for disk brake applications by simply removing the residual valve in the rear (front brake) outlet. Technically they work; but the rear resevoir needs topping off as the front pads wear.