I recently became interested in combination type pairs (U.S.) which have been touched upon in a couple scattered threads on the forum. I thought I would bump this old thread of Sc. 499 (type I) and 500 (type Ia) as there are combination pairs which show the two side by side after which the OP asked.
So, just to recap, Sc. 499 was the common Type I variety of this stamp. More than 300 plates were used in the printing of the billions of 499. But there was a slightly modified experimental transfer die made with which plates 10208 and 10209 were produced which Scott recognized years later in 1938 as type Ia Sc. 500 which is only from those two plates.
But during the production of Sc. 500 (type Ia) plate 10208 was touched up in the lower right pane with re-entries at positions 95 and 96. The experimental die was too large for single re-entry so the BEP used a single subject type I die! Position 95 is immediately above the plate number which marks it for attention IF the plate number is still attached.
There was an example in Siegel's recent sale of the Field collection of a combination type variety pair where the type Ia Sc. 500 is on the left in position 94 showing the stronger toga button lines etc., and a type I Sc. 499 with somewhat weaker lines is at the right in position 95 above the plate number.

Kind of an esoteric item which resists a quick, elevator-speech explanation, but there it is. I will admit I felt a certain imperative during the bidding for it because a some time ago I had acquired a plate number single of the Sc. 500 which was Dr. Hinrichs' and rather than 10209 it shows the same 10208 plate number, from a side position though.

I hope this is of interest and that my explanation is intelligible....
- Jonathan