Very Interseting , on 799 the B is on the document (works order form) but not stamped on the UFM... but the timing cover is an early one, with raised oil galley ways as shown on magazine where its road test was published... the 7 th of January 1949 (probably tested one month or a couple of weeks ago)
On this same magazine (Moto Revue 7th, & 8 th of January 49) there are the new models shown in the London show : so there is a picture of a Black Lightning with Girdraulic forks BUT the early timing cover as well...on page 4. (Comet with Girdraulic as well on the same page)
And a picture of Rollie Free (Roland F Free)on page 15 and comments on his record .