wiper arm

Lucas and Wipac were the main suppliers of screen wiper products to the British motor industry in the 70s, and both made wiper arms in all lengths from around 6 up to about 20 inches or so, in one inch increments. So you don't need to know the part number or which other cars used the same item, you just need to know the nominal length, and the easiest way to get that is for someone with a standard Bug to measure theirs and post the result. If you end up with an arm that seems not to fit too well, just get another one an inch shorter or longer as the case may be. There are modern equivalents widely available, and a fair bit of NOS too, on eBay and with the usual classic spares suppliers. As far as I know the splined end was a standard size across the industry, so there was probably not a single 'original' part, Reliant would have bought whichever supplier's batch was cheapest on the day - unless they had some sort of tied supply agreement with one of the main firms, but given the production volumes that seems highly unlikely, more likely they'd just have bought in a new batch when the stock was getting low.