I've spent a few happy hours going through my library looking for pictures of these vehicles in traffic. My interest is Scottish railways, so I have next to no Western Region books to look through. In case anyone is interested, here are the locations of the pictures I have found. They are arranged from the earliest date, as the introduction of these vans on Scottish traffic is what sparked my interest.
Steam Memories North East Scotland (33) 1950 fish train to the south hauled by B1 61323 and unidentified B12. INSIXFISH is 11th vehicle in train, coupled to other 6 wheel vans, 5 total. (BW) Same pic in BR Steam in Scotland (O'Hara) #593.
British Railways Steaming on the East Coast Mainline p65 (119) B16 61416 at Pilmoor station circa 1955. INSIXFISH is the first vehicle in the train.
BR Steam in Scotland #579 Eyemouth 11th June 1957. INSIXFISH in company with other fish vans, including 2 ex-LMS 6 wheelers (BW)
Scottish Railways Then and Now p104 80011 at Fraserburgh Junction on 12th June 1959. INSIXFISH appears to be the penultimate vehicle in a rake of Blue Spot vans. (BW)
Scottish Steam Days Remembered p48 46201 Princess Elizabeth on a lightly loaded express fish working near Perth in June 1962. INSIXFISH is 4th vehicle in train. (Colour)
Lanarkshire's Last Days of Steam p16 46236 City of Bradford on up fish near Mossend Yard on 1st May 1963. INSIXFISH appears to be 11th vehicle in train. (BW)
Pic of INSIXFISH at Thornton Junction (possibly a cripple?) on 16th July 1963. (BW) https://flic.kr/p/xMh7E4
Steam Finale Scotland p142 44986 at Dumfries 16th May 1964. INSIXFISH partially visible in background.
BR Steam in Scotland #594 Black 5s 44998 & 45259 near Kinnaber Junction on 6th July 1964. 2 INSIXFISH vans coupled together in consist. (BW)
Last Days of Steam in the Scottish Highlands p32 (Undated) D2432 at Greenock Ladyburn shed with a short mixed goods. INSIXFISH is 3rd vehicle in rake of 12t van, carflat, 2 x lowfit and conflat. (BW)
John Boyes pic of Fraserburgh shows an INSIXFISH with open doors in the distance. (Undated) (Colour) https://flic.kr/p/VbZn13
I hope this is useful for other modellers of these vehicles, and many thanks to Ian (and Justin) for a really excellent kit.