Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| Name | Map colour | First operated | Type | Length | No. Sta | Current Stock | Future Stock | Trips per annum (×1000) | Avg. trips per mile (×1000) |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| Bakerloo line | Brown | 1906.0 | Deep Tube | 23.2km 14.5 mi | 25.0 | 1972 Stock | N/A | 111136.0 | 7665.0 |
| Central line | Red | 1900.0 | Deep Tube | 74 .0 km 46 .0 mi | 49.0 | 1992 Stock | N/A | 260916.0 | 5672.0 |
| Circle line | Yellow | 1871.0 | Sub surface | 27.2km 17 .0 mi | 36.0 | C Stock S Stock | S Stock (Currently replacing C Stock) | 114609.0 | 4716.0 |
| District line | Green | 1868.0 | Sub surface | 64 .0 km 40 .0 mi | 60.0 | C Stock and D78 Stock | S Stock from 2014 | 208317.0 | 5208.0 |
| Hammersmith & City line | Pink | 1864.0 | Sub surface | 25.5km 15.9 mi | 29.0 | C Stock S Stock | S Stock (Currently replacing C Stock) | 114609.0 | 4716.0 |
| Jubilee line | Silver | 1979.0 | Deep Tube | 36.2km 22.5 mi | 27.0 | 1996 Stock | N/A | 213554.0 | 9491.0 |
| Metropolitan line | Purple | 1863.0 | Sub surface | 66.7km 41.5 mi | 34.0 | S Stock | N/A | 66779.0 | 1609.0 |
| Northern line | Black | 1890.0 | Deep Tube | 58 .0 km 36 .0 mi | 50.0 | 1995 Stock | N/A | 252310.0 | 7009.0 |
| Piccadilly line | Dark Blue | 1906.0 | Deep Tube | 71 .0 km 44.3 mi | 53.0 | 1973 Stock | N/A | 210169.0 | 4744.0 |
| Victoria line | Light Blue | 1968.0 | Deep Tube | 21 .0 km 13.3 mi | 16.0 | 2009 Stock | N/A | 199988.0 | 15093.0 |
| Waterloo & City line | Turquoise | 1898.0 | Deep Tube | 2.5km 1.5 mi | 2.0 | 1992 Stock | N/A | 15892.0 | 10595.0 |
Question:
When 4744 is the avg. trips per mile (x1000) what is the current stock?
Answer:
1973 Stock