Why the Sheffield? That's a guitar speaker specifically designed to mimick the low-wattage Celstions used in Marshall amps in the '60s and '70s.
If it's a Peavey MK III or MK IV in the 400 series, they had several cabinets they sold at the time. It would hav either generic speakers, Scorpions, or Black Widows depending on the specific cabinet, the year, and whether it was upgraded to BW or not. BTW, my personal preference for Peavey bass speakers is the Scorpion. Not as much power handling as the BW, but it's a lot punchier and less pillowy on the low end.
They made a very nice an portable for the time 1x15, various 2x15 cabinets, some 1x18s, and the 1810/1820. These last two had a single 18" BW and a pair of 10" Scorpions, a crossover with bi-amp input jacks. There was also a 3620 with two BW 18's. Big and heavy... My favorite Peavey cabinet of all time came a bit later- it's the 1516 which had a single 15" speaker and a pair of 8's. Similar crossover network as the 1820, but much more portable and to my ears much much better sounding- not nearly as muddy and woofy as the 18"s.
If the amp was made in 1973, neither the Scorpion nor the Sheffield were in the line then.
jte