This is the number tag attached to an original MGA Twin Cam radiator. (Photo compliments of Neil Burnside). The part number as found in the Service Parts List is ARH121, but may be stamped on this plate as ARH0121. "039" would be the production date, 3rd week of 1959, while trailing digits (00298) would be a serial number. This is the same (blank) plate as used on radiators for early MGA pushrod engined cars (see article CO-100), just the different part number stamped for the Twin Cam radiator.

Very early production Twin Cam radiator was part number ARH0098. Part number ARH0121 was introduced sometime after (c)801, about November-December 1958 (data collected by MGA Twin Cam Enthusiasts Group). Some examples of original Twin Cam radiator tag numbers:
806 ARH0098 00071
039 ARH0121 00076
029 ARH0121 00099
029 ARH0121 00106
029 ARH0121 00126
039 ARH0121 00294
039 ARH0121 00298
039 ARH0121 00351
039 ARH0121 00372

Here is the research end and my best guess:

ARH98 Block assembly -radiator (Twin Cam)
From "The MG Series MGA Service Parts List (1500)"
1st Edition, 3rd Printing December 1958 with Twin Cam supplement to the MGA 1500 SPL

ARH121 Block assembly -radiator -Twin Cam
From "The MG Series MGA Twin Cam Service Parts List"
SPL Version 2 and Version 3

So it looks like ARH98 was the part number from start of production, later changed to ARH121. If this was changed due to inadequate cooling, it was caught fairly early in production (maybe after the first 500 cars). Otherwise there is no specified date or chassis number for the change. But given a choice when acquiring a new radiator, ARH121 would be the preferred later style, with ARH98 being the early one for concours show.

I suspect the leading digits to be date of manufacture (kind of a guess) with the trailing digits being the serial number.
806 ARH0098 00071 -- 1958 6th week --- (3 prototypes before 1st production car in April 1958)
039 ARH0121 00372 -- 1960 39th week -- (Last MGA Twin Cam in May 1960, so more likely 3rd week or 3rd month 1959).