History of (Ealing - Hayes) route 274
here.
To expand on what
@greenline712 has said -
Yes, the blind is the right shape to be used in most London Transport single deckers before the Leyland Nationals, and Southall Garage had an allocation of SMS buses from the early 1970s onwards.
274 went to one person operation (with double deck DMSs) in 1977, and withdrawn in 1990, but the SMSs were withdrawn by around 1980, so the blind almost certainly dates from the late 70s and would have been made for an SMS.
There were a few LT routes where single deck buses could substitute for double deckers (and a very few vice versa) and there was more flexibility over this from around the late 70s / early 80s (I assume this was part of some deal with the union.)
A brief search on Flickr hasn't come up with any SMSs on the 274, although this of course doesn't prove it never happened. There are one or two books on London Transport 'odd workings' but I don't have a copy to hand.
It could be that it was included on blinds but never used because an idea was dropped for one reason or another - there are a few examples of this over the years. I think there were a few examples of blind sets made showing routes that ended up never existing or never operating from that garage (although I can't remember the details.)