The double convex and plano concave come as a matched set as they're made of glass with differing refractive indexes. Some of the later replacements (made by Copyground in High Wycombe incidentally who are now out of business........) were cememted together because........ lets just say that it suited his needs with the glass he was using which gave a corrected figure.............. BUT, you could try to use a second hand double convex and having said that, it COULD be your lucky day because as I said above, generally, it's always the outer that breaks when the cack handed clown drop them and I have a couple of the inner double convexes that you can try. But in order to save having to explain how you do it correctly and optically, you'll have to send me your OG housing and when I'm in the mood, I'll see if either of those I have will mate up correctly with what you've got.

Not saying that it WILL work but it has been known to and it'll save you the £160+ for a new lens set