I went for http://m.ebay.com/itm?itemId=400748421329 ... very bright with the lens on the front and fits in the standard sideight bulb holder in the SVC 5.75" headlight.
I think the 11 in the part number refers to 1.1mm preset gap.
So ... leave alone and fit.
For those of us brought up on Mini and Cortina engines ... it looks way too big !
BS538 is only part of the code. In the world of hosereels we used BS538.381C which was cherry red ... so there are probably many variations of shades under BS538
On my 2002 car .... there are separate fuses for the LH and RH sidelights .... 6th and 7th fuses down from the top. One is 5A and one is 7.5A .... think it does rear number plate lights too ?