When Lucas says "lamp", they mean the entire assembly, not just the bulb. The Lucas bulb numbers are 382 for single filament (later cars with separate rear turn signals) and 380 for dual filament (earlier cars with combined turn signal/tail lights).
For the single filament, 1156 is probably closest to the original bulb spec. But if you're interested in a bit more brightness, a 2396 gives off about 20% more light and draws only slightly more current (about 5%). The tradeoff is shorter life, 400 hours vs 1200 hours, but that shouldn't be an issue for turn signals.
The equivalent for the dual filament is 2357.
They can be a bit hard to find, but I have found them at Pep Boys, Kragen, Autozone, etc.