This is 347 fewer tickets than the competition entitlement and 1,085 fewer than Liverpool got when they played in an FA Cup fourth round replay in February.
After meeting with MUST committee members last year, United officials acknowledge the importance of Safety Advisory Group (SAG) meetings – where some ticket allocations are set – and agreed to start attending them when necessary.
It is understood that United contacted West Ham around March 14 about the away fans’ allocation for the match.
United were unable to contact SAG organisers directly as is it protocol to make such requests via host clubs - in this case West Ham.
A United source said: “A member of staff personally requested to be invited (to the West Ham SAG meeting) via the ticket office manager at West Ham, but we were given radio silence for four days, only to then be told that the SAG meeting had taken place and the 4,905 allocation was decided.”
At the SAG meeting, on March 15, representatives from West Ham, Newham Council and the emergency services agreed the away-end layout.
They said the front two rows of the upper and lower tier of the away end cannot be sold and neither can two columns of aisle seats or the block closest to the West Stand.
United believe that, without these reductions, the allocation would have been approximately 1,000 higher – similar to Liverpool's.
The presence of a United member of staff at the SAG meeting could arguably have led to a higher allocation as they could have suggested ways of tackling potential safety problems not open to other officials.
Such measures might include putting United stewards in the away end and sending letters to all ticket holders asking them to be aware of ground regulations.
United have earned ticket increases or prevented ticket reductions at other away games this season by suggesting such measures.
What has left officials frustrated on this occasion is that they were not given the opportunity to make their case and fans have potentially lost out as a result.
MUST asked West Ham United for a comment but has so far not received a response.
Responding to a fan's query, West Ham's supporter liaison officer Sascha Gustard-Brown said: "This particular meeting is organised and chaired by the local council, one to which we are invited to.
"It is not a meeting we lead on, or ‘own’ and as such we do not have any control over who is/is not invited."
On behalf of West Ham SAG, a Newham Council spokesman did not comment on why United were not informed of the meeting.
A statement reads: “The issue of the seat allocation for the forthcoming cup tie between West Ham United and Manchester United was discussed at a meeting of the SAG on March 15.
“This group includes representatives of West Ham United and the emergency services.
“Measures to help ensure the safety of those attending the match were agreed which the club are now taking forward.”
MUST have written to the Premier League about the issue.
The Premier League has spoken to officials at the Sports Grounds Safety Authority (SGSA), which provides advice to SAGs on allocation-setting issues.
While no SGSA representative was at the March 15 meeting, they are expected to attend future meetings.
The Premier League has asked SGSA officials to ensure United are informed of any West Ham SAG meeting at which the away ticket allocation for the May 10 league game will be set.
A Premier League source said: “It doesn’t make sense for representatives of the visiting club to be excluded.”
When similar issues have arisen in the past, the FA, which governs the FA Cup, has said the away allocation-setting procedure is the responsibility of the local SAG.
MUST has written to the West Ham SAG chairman to raise concerns about the matter.
MUST has also submitted Freedom of Information Requests for emails and meeting minutes to establish what were viewed as the key differences between West Ham's matches against Liverpool and United.