The original Austin had a two-tone dial as shown in this 1950 catalog snip.
Then the 1952 catalog shows the Austin without the rectangular area and with markers as well as numerals on the dial.
The Austin utilized the 14/0 sized, 17 jewel 980 movement. The Austin B used the 12/0 sized, 17 jewel 752 movement introduced in 1952. Since the movements are different, the case backs are different. The dials are also not interchangeable. However the crystals are the same as are the other general dimensions of the case.
Here's a before and after shot of an Austin I once restored. Here's before...
and here's after...
And here is an Austin-B that I used to have. I sold it as I think the Austin B is a little boring compared to it's older sibling.
The Austin is also one of several watches in the early 1950's that was available in both yellow and white gold filled. Here's an example of the white Austin.