Motospecs can supply the new roller top - part no. A50280RM - branded as 'Newcom', where the 'Kirby Bishop' writing was.
Newcom Industries Pty Ltd, is the Pedders parent company, and who make the Pedders springs, rebuild their steering componentry, etc.

http://www.newcomindustries.com/   The Newcom roller top, also comes as a part of the Motospecs full steering box rebuild kit - part no. GSB-3021

Before they were available, I got some bush tops retrofitted with a bearing, by searching the Timken catalogue at Bearing Wholesalers. The existing bronze bush, needs to be machined out on a lathe, and the recess where the bush was, needs to be center bored to open it up slightly.
The bearing I used, was a Timken full complement torrington race needle roller bearing, part no. 32208.   http://www2.timken.com/timken_ols3/Bearings/GST/WEB_GST2008/GST_BrgSrchDataPlus.asp?BrgMainType=TRB&BrgSubType=TS&Cone=33208&Cup=33208&Series=33208&PNum=33208&Unit=false&BrgImage=Images/TRB-TS.bmp&DataPlusStr=0.36::1.68::40::80::32::143::37.1::144::321.5::51::47::2.8::2.4::25::1.5::70::76::-10.9::9.316::6.684::2.898::0.4178::0.5822::-::&DataPlusStr1=&DxfFileStr=8350**37100**1.63**5120**22800**32200**143000**1.61**0.73**1.5748**40**3.1496**80**1.2598**32**1.2598**32**0.9843**25**0.06**1.5**0.06**1.5**14500**64700 http://www.timken.com/EN-US/products/bearings/productlist/roller/needle/Pages/drawncup.aspx   The Newcom roller top, is NOT a full complement torrington race bearing.
The bearing is branded NTN, with a part no. of SCE 208A

Full complement, means that each roller abuts the next roller, all the way round the bearing.</p>
Caged needle roller (torrington) bearings are cheaper, and have half as many rollers - which are spaced apart by a plastic insert (cage).   I have a NOS Ford top, which came from the factory with the torrington race bearing, (they were obviously being fitted at a later stage - XG?)
- Part no. XA3580AKT.
It's branded 'Kirby Bishop', and has a full complement torrington race bearing - with an outer cup part no. B208 (could be timken branded)
It must be a late production item, as the wording and finish of the top, is very average - hence the sand casting mould must have been very worn out.

The Newcom tops are cast very tidy, so they must've purchased and refurbed (or copied) the mould, then rebranded it too.

Interestingly, the four bolt holes are tapered holes.
BUT......
the holes in the Kirby Bishop top, are tapered with smaller opening on the inside, and larger on the outside.
Whereas the four bolt holes on the Newcom top, are tapered opposite - with the larger end on the inside.

One of them, has had the holes cast from the opposite end - ie, one top has been cast in a mould facing up, and the other, cast facing down.
How (or if) that would affect the fit, I dunno. We'd have to compare it with a couple of tops, off some non-rebuilt steering boxes, to get the best two out of three or something.

On both tops, the holes are cast as part of the mould, they're not drilled afterwards.

Note the awesome quality control with the NOS Kirby top, in the last pic......