Right - in the office and I have been through the archives of the Yahoo BR wagons group. Firstly, from
Modern Railways 1963. In the April there is an article about 'House Style'. There are pictures of a conflat - this picture, I suspect:
BR Conflat A wagons and containers FAV ZYV ZSV ZRV ZXA ZVV | B504777 Conflat A BD46577B Experimental livery � Paul Bartlett Collection w - with the white square displaying a figure 4 in the centre, and also a standard 12 ton vanfit also with the 4 in a box. There is plenty of discussion about the new colour schemes and to which types of wagon would be repainted, but no comment on the actual box. Of interest is that long wheelbase palvans built at Ashford for Ford traffic in 1963 carried a box with the figure '2' in it, though ex-works pictures of similar vans show a figure '1'. Compare these two pictures:
BR Ford Palvans VQB RRB ZEB ZRW Diag 1/235 | B787044 - work's picture, figure '1'
BR Ford Palvans VQB RRB ZEB ZRW Diag 1/235 | B787047 - in service picture, figure '2'
In
November there was an article about a new stricter system of wagon control on BR, it has one very interesting statement, that for
Control purposes the wagon stock has been divided into four classes. That these numbers refer to one of the proposed classes is an interesting but, erroneous theory; the 21 ton hoppers have the number 5 in their boxes for a start, but read on...
Ian Fleming supplied information from
Minute 5821 of the Wagon Standards Sub-Comittee, dated 28.2.63 and found at the NRM. It details proposed categories 1 to 8, descending in speed from 75mph to 35mph - this ties in quite strongly with occasional photographic evidence such as the vehicles above, and suggested that the figure was, or would have been, painted in the box that in practice more usually contained the 'XP' in the post-1964 [sic - it was current for some new build stock from at east 1963; see the Palvan images linked to above] lettering style.
There is, however, no mention of any 'unclassified' category, which is possibly what an empty box signified at that point. Although only speculation on Ian's part, it could be that an empty box signified nothing more than that the vehicle *wasn't* XP-rated. I think this is extremely likely, at least, in original intent.
The following appeared in LMR WON [Wagon Operating Notices] in September 1963 (information from Mike Hollick). I have highlighted the appropriate bit.
NOTICE TO STAFF.
Limited speed of all four-wheeled vehicles of 10 feet wheelbase or less, and reduced maximum speed of Class 4 trains.
Until further notice all four-wheeled vehicles with a wheelbase of 10 feet or less are subject,to a speed restriction of 50 m.p.h. ,
If it is necessary for any such vehicle to be conveyed on a Class 1, 2 or 3 train, the Guard must advise the Driver and instruct him not to exceed 50 m.p.h.
Class 4 or 4+ trains must not exceed 50 m.p.h. at any point unless indicated in the timetable by a "club" symbol, in which case a maximum speed of 55 m.p.h, (Class 4) or 60 m.p.h. (Class 4+) will be permissible. Should it be necessary for a train so indicated to
carry a four-wheeled vehicle with a wheelbase of 10 feet or less the Guard must instruct the Driver not to exceed 50 m.p.h.
Painting of Freight Stock and Non-bogie Coaching Stock.
It is the future intention that all freight stock and non-bogie coaching stock will be marked to Indicate the maximum speed at which it may run. It may have been observed that a number of vehicles have already appeared in service bearing a white numeral in a white lined square, and without the symbol "XP" where applicable.
Full instructions on this subject will be published in due course. In the meantime it should be noted that the Numerals "1" "2" or "3" are equivalent to "XP" marking.
(21-9-63)
And finally, we have this document, showing that the idea was still live in 1967 but had not been generally acted upon at that date and, if photographic evidence is anything to go by, never was.
If anyone is still reading at this point, all this points to the two 21 tonners being repainted in 1963 (both wagons were constructed in the 1950s: B429561 from Gloucester C&W Co., lot 3159 of 1958, for example) and that this, taken with my previous post suggests with a very high degree of probability - and rarer still, in the world of wagon discussions, with proper contemporary references! - that the photo was taken in 1963.
Adam