John Budgen has stanchions, bushes etc listedhttp://www.johnbudgenmotorcycles.co.uk/ ... alogue.htm
Paul Goff has 'equivalent' BSA parts (although the bushes are in his 'upgrade' kits, not listed separately).
All Ariel stanchions/bushes are physically interchangeable but the bushes were lengthened during the 1950's and also the positioning of the damping holes. As far as I'm aware 1950's BSA stanchions/bushes are similarly physically interchangeable with Ariel components but again the damping holes can be different.
I don't know if the Ariel and BSA changes were the same, or made at the same time. However, as long as the set of components is a matched spec (ie damping holes in the same place on both legs and the appropriate short/long bushes) I'm pretty certain they'll work fine on a '51 bike.
(I notice that Goffy seems quite expensive - I've spoken to him at the Shepton jumble a few times and one year he was having a good moan about how long it had taken him to get properly-sized stanchions and bushes for a front-end rebuild on his BSA so possibly/hopefully his prices reflect higher quality components!)
'55 Huntmaster, '56 VH, ' 51 VH, '80 R100RT, '00 Sprint ST (but all those Ariel parts can only make one running bike...)