Installing gear cable guide on 80s Holdsworth I have this Holdsworth frame that I got repainted and am rebuilding with mostly modern parts - I recently realised that I'm not quite sure how to run the derailleur cables past the bottom bracket.

I have seen bikes from this era with brazed on above-BB cable guides, but this frame doesn't have them.

I then remembered my Btwin has a plastic under-BB guide, so I got one of those but realised the hole in the Holdsworth's BB cage looks too small to have any sort of screw in it, and looks more like a drainage hole.





(Also, even though the guide says "screw on type", it looks quite different from the screw on one on my Btwin, which has a split plastic nub which wedges into the BB cage hole when the screw is tightened. I'm really not sure how this one I got is supposed to be fastened - I looked up some diagrams on Shimano's site but they didn't help) 

Does anyone have any advice on how I should go about guiding the gear cables? Thank you!
The little hole looks a similar size to the one in my early 90's steel frame BB. Mine is threaded, perhaps yours is too. Maybe it didn't get masked off by the painter and the thread is under the paint. You could ask the painter if they remember taping it or not. You could check for threading and possibly tap the hole yourself if if isn't already.

You can also get cable guides which have a little solid plastic nub which goes into the hole and there is no screw, it is held in place by cable tension. I'll have a look for an image or something but might be a needle in a haystack. Chances are an LBS or framebuilder/painters will have some miscellaneous ones, you could ask around in person and see if you can find one to suit your frame, designs vary.

Other suggestions, from here include glue (!) and double sided tape to position the guide before cable tension takes over. I don't know how they would affect the paint long term.