On 10/17/2010, Adam Prince of Honeoye Falls, New York, wrote:
"Replaced both rear wheel cylinders. The cheaper ones from Moss Motors were mismanufactured with too thick of a flange. Filed the flange down to fit".

Moss Motors offers two different rear slave cylinders. The more expensive one is 180-630 - Lockheed/Delphi. The less expensive one is 180-075 - aftermarket (made in Taiwan) and labeled "Classic Gold" (as Moss special manufacture). 180-075 is the one with the problem.

In photo above, space between the two white lines is a bit too narrow so the part does not fit on the brake backing plate. Photo below left shows the part that doesn't fit at top, and another part that was removed from the car (did fit). Red arrows point at the area with different dimensions. Photo below right shows Adam's solution, filing the surface that mates against the outboard side of the backing plate. Seems to me it may be easier to file the opposing surfaces (inboard side of the backing plate) across the ends as pointed out by white arrows in photo above.

Moss Motors has been notified of the problem (October 2010). They report there has been one other similar complaint in 2010. No telling yet if these are rare cases or if people are fixing the parts without reporting the problem.

Addendum September 2011: -- (Photos from Michael Kelly in Taylorsville, KY)

Here is another approach showing filing of the smaller opposing surface at the ends only (before left, after right).

Pictures below show the extent of motion where the cylinder slides in the backplate slot (paint removed).

Addendum April 6, 2016:
I had not received any prior word about this, but today I installed a couple of Moss Motors sourced MGA rear slave cylinders, and they fit well on the first try. Problem has been banished, apparently.

Or maybe not.
Addendum March 26, 2017:
On 3/26/2017, Roger Luckenbill wrote:
"Just finished installing Moss rear wheel brake kit and had to file surfaces on both the wheel cylinders to allow unit to slide in slot on backing plate. Apparently not yet resolved. I wonder if people are installing these cylinders not realizing they must slide freely in the slot".

On 3/30/2017, Roger Luckenbill wrote:
"Just checked the invoice. It was dated 8/05/16. Part #180-648".

Addendum April 25, 2017:
On 4/25/2017, Eric Glomstad wrote:
"I received an email today saying that 'No Credit' would be given because the parts were test fit. They offered to send the parts back. - Eric

Moss Motors has known about this problem for at least 6-1/2 years, and they are refusing to fix it. Apparently not enough people are sending the parts back for refund, so it stands to reason that these are perfectly acceptable parts for the retail market. We vote with our money, and apparently not enough people care to vote.